Insert spring into lock for unlawful entry (8)
I believe the answer is:
trespass
'unlawful entry' is the definition.
(I've seen this in another clue)
'insert spring into lock' is the wordplay.
'insert spring' becomes 'pas' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'into' is an insertion indicator.
'lock' becomes 'tress' (both can mean a piece of hair).
'pas' inserted within 'tress' is 'TRESPASS'.
'for' is the link.
(Other definitions for trespass that I've seen before include "Encroachment" , "Go where one shouldn't" , "Wrongfully enter another's land" , "Entry without permission" , "Illegal entry" .)