Enter unlawfully, inserting spring into lock (8)
I believe the answer is:
trespass
'enter unlawfully' is the definition.
(I've seen this in another clue)
'spring into lock' is the wordplay.
'spring' becomes 'spa' (spa can mean a mineral spring).
'into' means one lot of letters goes inside another.
'lock' becomes 'tress' (both can mean a piece of hair).
'spa' going within 'tress' is 'TRESPASS'.
'inserting' acts as a link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for trespass that I've seen before include "sin in the eyes of the church" , "Enter another's property without permission" , "Enter someone's property unlawfully" , "Enter the Forbidden Land" , "Enter another's property unlawfully" .)