Some extra derequisitioning done by merchant (6)
I believe the answer is:
trader
'merchant' is the definition.
(trader is a kind of merchant)
'some extra derequisitioning done' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'TRADER' is hidden within 'extra derequisitioning done'.
'by' is the link.
(Other definitions for trader that I've seen before include "One buys and sells" , "Merchant ship; dealer" , "He buys and sells goods" , "Red tar (anag)" , "One in business" .)