Cause suffering to company during sudden upturn (7)
I believe the answer is:
scourge
'cause suffering' is the definition.
(scourge can mean torment or afflict)
'company during sudden upturn' is the wordplay.
'company' becomes 'co' (abbreviation).
'during' is an insertion indicator.
'sudden upturn' becomes 'surge' (a surge is a sudden rise).
'co' inserted inside 'surge' is 'SCOURGE'.
'to' is the link.
(Other definitions for scourge that I've seen before include "Flog, punish" , "torment" , "Plague; whip" , "Punish severely" , "Flagellate" .)