Firm interminably exploiting relative (6)
I believe the answer is:
cousin
'relative' is the definition.
(I know that cousin is a type of relative)
'firm interminably exploiting' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'interminably' means to remove the last letter.
'exploiting' becomes 'using' (I've seen this in another clue).
'using' with its final letter taken away is 'usin'.
'co'+'usin'='COUSIN'
(Other definitions for cousin that I've seen before include "Aunt/uncle's child" , "person once removed?" , "Child of a parent's sibling, say" , "Strange US icon is a relative" , "Kinsman, generally (archaic)" .)