Castle not wanting for lock (5)
I believe the answer is:
tress
'lock' is the definition.
(both can mean a piece of hair)
'castle not wanting for' is the wordplay.
'castle' becomes 'fortress' (I've seen this before).
'not wanting' is a deletion indicator.
'fortress' with 'for' removed is 'TRESS'.
(Other definitions for tress that I've seen before include "Rests (anag.)" , "Bit of hair" , "Curl - lock" , "Long piece of hair" , "is a bit of a shock" .)