Unpleasant person mostly depressed by employment (5)
I believe the answer is:
louse
'unpleasant person' is the definition.
(louse is a kind of unpleasant person)
'mostly depressed by employment' is the wordplay.
'mostly' means to remove the last letter.
'depressed' becomes 'low' ('low' can be a synonym of 'depressed').
'by' says to put letters next to each other.
'employment' becomes 'use' ('use' can be a synonym of 'employment').
'low' with its final letter removed is 'lo'.
'lo'+'use'='LOUSE'
(Other definitions for louse that I've seen before include "Insect -- nasty character, undeserving of respect" , "Nit" , "scum" , "Wingless parasitic insect" , "25 [PARASITE]" .)