Cries when boss is nasty (4)
I believe the answer is:
sobs
'cries' is the definition.
(sobbing is a kind of crying)
'boss is nasty' is the wordplay.
'is nasty' is an anagram indicator.
'boss' anagrammed gives 'SOBS'.
'when' is the link.
(Other definitions for sobs that I've seen before include "Weeps audibly" , "Weeps noisily" , "Cries noisily" , "convulsions" , "Cries loudly" .)