Sukkha is basically the opposite of Dukkha, it means happiness. Dukkha means pain, Sukkha means happiness, pleasure. That is what it means. Can we hold it or is it better to work on letting it go? It depends on where the Sukkha is coming from, it is not clear to me here whether this is just a normal happiness, Sukkha, like having your favorite Pizza or so, or whether it is a happiness like I have mentioned before like giving, the happiness that comes from giving. If it is a happiness arising when you are reflecting on your good past Kamma - hold it for a while, that is your reward. You'd like to hold it for a while, but you also have to know that it is going to pass, so you are not going to hold on to it forever. But you definitely don't want to let it go very quickly. That is when you are reflecting on your good Kamma. Now imagine you are trying to do the Compassion/Lovingkindness meditation. And you are going along fine, thinking about a few people, and then all of a sudden your mind drifts off to your good Kamma and you are happy about your good Kamma. Hindrance - note it, let it go, get back to the meditation subject, so that is important, too. When the Sukkha is coming at the wrong time, we actually don't want it.
I was amazed once listening to someone tell me and a lot of other people about how much Sukkha, happiness, they had when they were doing Compassion meditation. Now this was not Compassion/Lovingkindness meditation, this was just Compassion meditation. Now think about this, we are just opening up the compassion, we are thinking of people's pain. We are wishing the pain was not there, we are just holding on to those thoughts, thinking of pain, wishing it was not there... thinking of pain, wishing it was not there... And this person was explaining how they got really happy and excited because their mind got super concentrated and they tripped off into a phenomena. They got excited and they were lost from the meditation, but they actually thought this was a great part of the meditation.
I thought to myself "no, no". There is no way I am going to think of some beggar on the street and get happy. There is no way I am going to think of some blind person and get happy. So this is an important point, if you have some Sukkha arise, investigate where is it coming from and is it appropriate at that time? If it isn't appropriate then we do want to let it go.