Sceptic, a non-drinker, he is inside (7)
I believe the answer is:
atheist
'sceptic' is the definition.
(I've seen this before)
'a non-drinker he is inside' is the wordplay.
'non drinker' becomes 'tt' (abbreviation for teetotaller).
'inside' is an insertion indicator.
'he'+'is'='heis'
'tt' placed around 'heis' is 'theist'.
'a'+'theist'='ATHEIST'
(Other definitions for atheist that I've seen before include "One who disbelieves the existence of gods" , "I don't believe" , "Unbeliever" , "Sceptic" , "Person who believes there is no God" .)