Very loud on my watch, becoming calm (4,3)
I believe the answer is:
cool off
'calm' is the definition.
(synonyms)
'very loud on my watch' is the wordplay.
'very loud' becomes 'ff' (musical abbreviation).
'on' says to put letters next to each other.
'my' becomes 'coo' (both are expressions of surprise).
'watch' becomes 'lo' (archaic exclamation meaning 'look!').
'coo'+'lo'='coolo'
'ff' put after 'coolo' is 'COOL OFF'.
'becoming' is the link.
(Other definitions for cool off that I've seen before include "grow less passionate" , "calm down" , "simmer down" .)