Key worker's jargon (4)
I believe the answer is:
cant
'jargon' is the definition.
(synonyms)
'key worker's' is the wordplay.
'key' becomes 'c' (musical key).
'worker' becomes 'ant' (worker is a type of ant).
'c'+'ant'='CANT'
(Other definitions for cant that I've seen before include "Hypocritical talk - sounds negative" , "lean over" , "make a lot pay?" , "Insincerity" , "tilt" .)