Plays a part in security, but needs to be soft (4)
I believe the answer is:
hasp
'plays a part in security but' is the definition.
I can't judge whether this defines the answer.
'needs to be soft' is the wordplay.
'needs' becomes 'has' ('have' can be a synonym of 'need').
'to be soft' becomes 'p' (musical abbreviation for 'piano', soft).
'has'+'p'='HASP'
Can you help me to learn more?
(Other definitions for hasp that I've seen before include "Fastening plate, clasp" , "Door latch" , "Hinged metal fastener" , "catch" , "Hinged fastener for door or lid" .)