Firm in rush to cause suffering (7)
I believe the answer is:
scourge
'cause suffering' is the definition.
(scourge can mean torment or afflict)
'firm in rush' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'in' means one lot of letters goes inside another.
'rush' becomes 'surge' (synonyms).
'co' put inside 'surge' is 'SCOURGE'.
'to' is the link.
(Other definitions for scourge that I've seen before include "Affliction - whip" , "Afflict" , "Whip, flog" , "Flog, punish" , "A cause of great trouble" .)