Disheartened attendant serves customer (4)
I believe the answer is:
user
'customer' is the definition.
(I've seen this in another clue)
'disheartened attendant' is the wordplay.
'disheartened' says to hollow out the word (remove centre letters) (the middle or heart is removed).
'attendant' becomes 'usher' (usher is a kind of attendant).
'usher' with its centre taken out is 'USER'.
'serves' acts as a link.
(Other definitions for user that I've seen before include "Right long-established" , "Druggie" , "Manipulator" , "Operator; drug addict" , "Drug taker" .)