Lets look at order of improtance when dieting down

1 Deit

2 cardio

3 fat burners 

Clen wil help to push these things further, but you want your foundation to be set, diet and cardio first. Then add fat burners like clen

As for building on clen, theres no point. The idea of off seaso is to eat enough food to grow. Using clen would burn up some of the cals that you couple be using for gains. If you find that you are getting a little too fat in the off season, reduce your carbs a little. Then maybe add in a small amount of cardio for 2 wls, then go back to the normal plan