Cries, when boss is upset (4)
I believe the answer is:
sobs
'cries' is the definition.
(sobbing is a kind of crying)
'boss is upset' is the wordplay.
'is upset' is an anagram indicator (I've seen 'are upset' mean this).
'boss' anagrammed gives 'SOBS'.
'when' is the link.
(Other definitions for sobs that I've seen before include "Given evidence of distress" , "showing distress" , "convulsions" , "Cries loudly" , "Is unhappy" .)