Make illegal entry — father’s breaking lock (8)
I believe the answer is:
trespass
'make illegal entry' is the definition.
(I know that trespass is a more specific form of the action make illegal entry)
'father's breaking lock' is the wordplay.
'father's' becomes 'pa's' (pa can mean dad or father).
'breaking' means one lot of letters goes inside another.
'lock' becomes 'tress' (both can mean a piece of hair).
'pas' placed into 'tress' is 'TRESPASS'.
(Other definitions for trespass that I've seen before include "Enter land illegally" , "Enter someone's property unlawfully" , "Go out of bounds" , "Commit a sin -- spa's rest (anag)" , "Encroachment" .)