Heard expressions of relief from circuit breaker (4)
I believe the answer is:
fuse
'circuit breaker' is the definition.
'heard expressions of relief' is the wordplay.
'heard' shows a homophone (sound like).
'expressions of relief' becomes 'phews' ('phew!').
'phews' is a homophone of 'FUSE'.
'from' is the link.
(Other definitions for fuse that I've seen before include "Tube or cord for igniting an explosive" , "Coalesce, melt together" , "Electrical fitting" , "'Blend, merge (4)'" , "Meld" .)