Believer a bit wet? Not the fellow being imprisoned (10)
I believe the answer is:
monotheist
'believer' is the definition.
(I know that monotheist is a type of believer)
'a bit wet? not the fellow being imprisoned' is the wordplay.
'a bit wet?' becomes 'moist' (I've seen this before).
'the fellow' becomes 'he' (he refers to a man).
'being imprisoned' means one lot of letters goes inside another.
'not'+'he'='nothe'
'moist' enclosing 'nothe' is 'MONOTHEIST'.
(Other definitions for monotheist that I've seen before include "I'm A Believer" , "Maybe Christian" , "Christian, for one" , "One who believes" .)