Charge for the firm's time (4)
I believe the answer is:
cost
'charge' is the definition.
(I've seen this before)
'firm's time' is the wordplay.
'firm's' becomes 'co's' (abbreviation for company).
'time' becomes 't' (abbreviation).
'cos'+'t'='COST'
'for the' is the link.
(Other definitions for cost that I've seen before include "Charge" , "Fee" , "estimate price" , "penalty" , "Outlay required to buy" .)