Worked hard to make a stewed concoction (7)
I believe the answer is:
sweated
'worked hard' is the definition.
'a stewed concoction' is the wordplay.
'concoction' indicates anagramming the letters.
'a'+'stewed'='astewed'
'astewed' with letters rearranged gives 'SWEATED'.
'to make' is the link.
(Other definitions for sweated that I've seen before include "Reacted to heat; exploited (labour)" , "(Of labour) ill treated and ill paid" .)