Force spring in lock in unlawful act (8)
I believe the answer is:
trespass
'unlawful act' is the definition.
(trespassing is an unlawful act)
'force spring in lock' is the wordplay.
'force spring' becomes 'pas' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'lock' becomes 'tress' (both can mean a piece of hair).
'pas' put within 'tress' is 'TRESPASS'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for trespass that I've seen before include "Transgress" , "Wrong" , "Enter another's property without permission" , "Sin (archaic)" , "Intrude on another's property" .)