Earnings are mine, working with firm (6)
I believe the answer is:
income
'earnings' is the definition.
(I've seen this before)
'mine working with firm' is the wordplay.
'working' is an anagram indicator.
'with' means one lot of letters goes inside another.
'firm' becomes 'co' (a firm is a company).
'mine' is an anagram of 'inme'.
'inme' enclosing 'co' is 'INCOME'.
'are' is the link.
(Other definitions for income that I've seen before include "Money coming in" , "Money from investments or for work" , "Money received for work or from investment" , "Wage" , "Remunerations, takings" .)