Pare while the abstainer is inside (7)
I believe the answer is:
whittle
'pare' is the definition.
(I know that pare can be written as whittle)
'while the abstainer is inside' is the wordplay.
'the abstainer' becomes 'tt' (I've seen this in other clues).
'is inside' means one lot of letters goes inside another.
'while' placed around 'tt' is 'WHITTLE'.
(Other definitions for whittle that I've seen before include "Pare thin shavings" , "Reduce - jet engine inventor" , "Reduce (with a knife)" , "Aeronautical engineer" , "Cut wood aimlessly" .)