Disheartened doorkeeper's employer (4)
I believe the answer is:
user
'employer' is the definition.
(thesaurus)
'disheartened doorkeeper's' is the wordplay.
'disheartened' means to remove the middle letters.
'doorkeeper' becomes 'usher' ('usher' can be a synonym of 'doorkeeper').
'usher' with its middle removed is 'USER'.
(Other definitions for user that I've seen before include "One who employs" , "One who works an appliance" , "Druggie" , "Drug addict (informal)" , "Drug-taker" .)