Exit needed for one unopened lock (6)
I believe the answer is:
egress
'exit' is the definition.
(I've seen this before)
'for one unopened lock' is the wordplay.
'for one' becomes 'eg' (I've seen this in other clues).
'unopened' means to remove the first letter.
'lock' becomes 'tress' (both can mean a piece of hair).
'tress' with its first letter removed is 'ress'.
'eg'+'ress'='EGRESS'
'needed' acts as a link.
(Other definitions for egress that I've seen before include "W ay out" , "Way to escape" , "Way to go out" , "Taking leave" , "Way out of serges" .)