Part of a lock, part with a spring (4)
I believe the answer is:
hasp
'part of a lock' is the definition.
(I've seen this before)
'part with a spring' is the wordplay.
'part' says the answer is hidden in the clue.
'HASP' is hidden in the letters of 'with a spring'.
(Other definitions for hasp that I've seen before include "Door latch" , "catch" , "This part of fastening device" , "Fastening plate, clasp" , "Hinged fastening" .)