Firm happy to manipulate accounts, say (4)
I believe the answer is:
cook
'manipulate accounts say' is the definition.
(as in 'cooking the books')
'firm happy' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'happy' becomes 'ok' (ok can mean happy or comfortable).
'co'+'ok'='COOK'
'to' is the link.
(Other definitions for cook that I've seen before include "work in the galley?" , "18th-century explorer" , "Prepare food" , "Capt. James --, explorer" , "New England Test captain" .)