Unreliable spear carrying fellow finally stops working in rep (11)
I believe the answer is:
salesperson
'rep' is the definition.
(as in a sales representative)
'unreliable spear carrying fellow finally' is the wordplay.
'unreliable' indicates an anagram.
'carrying' means one lot of letters goes inside another.
'fellow' becomes 'les' (Les is a man's name).
'finally' becomes 'son' (I am not sure about this - if you are sure you should believe this answer much more).
'spear' anagrammed gives 'saper'.
'saper' going around 'les' is 'salesper'.
'salesper'+'son'='SALESPERSON'
'stops working in' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for salesperson that I've seen before include "Retail worker" , "Shop attendant" , "Shop server" , "One must try to improve business" , "Shop assistant" .)