Father's participating in very small sin (8)
I believe the answer is:
trespass
'sin' is the definition.
(I know that sin can be written as trespass)
'father's participating in very small' is the wordplay.
'father's' becomes 'pa's' (pa can mean dad or father).
'participating in' means one lot of letters goes inside another.
'very' becomes 'tres' ('very' in French).
'small' becomes 's' (abbreviation - e.g. clothes size).
'tres'+'s'='tress'
'pas' going within 'tress' is 'TRESPASS'.
(Other definitions for trespass that I've seen before include "sin in the eyes of the church" , "Commit a sin -- spa's rest (anag)" , "Wrong" , "Action of intruder" , "Entry without permission" .)