Small ringing sound is a pain (5)
I believe the answer is:
sting
'pain' is the definition.
(sting is a kind of pain)
'small ringing sound' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'ringing sound' becomes 'ting'.
's'+'ting'='STING'
'is a' is the link.
(Other definitions for sting that I've seen before include "Sharp tingling pain" , "Entrapment operation; sharp pain" , "Painful wound - confidence trick" , "Sudden pain - swindle" , "Bee/wasp wound" .)