Spring inside lock for illegal entry (8)
I believe the answer is:
trespass
'illegal entry' is the definition.
(I've seen this before)
'spring inside lock' is the wordplay.
'spring' becomes 'spa' (I've seen this before).
'inside' is an insertion indicator.
'lock' becomes 'tress' (I've seen this before).
'spa' put into 'tress' is 'TRESPASS'.
'for' is the link.
(Other definitions for trespass that I've seen before include "Intrude, encroach" , "Encroachment" , "Unlawful entry to someone's property" , "Intrude on another's property" , "Go out of bounds" .)