Commercial lines are damaging … (7)
I believe the answer is:
adverse
'are damaging' is the definition.
(as in adverse weather)
'commercial lines' is the wordplay.
'commercial' becomes 'ad' (abbreviation for advertisement).
'lines' becomes 'verse' (lines of poetry).
'ad'+'verse'='ADVERSE'
(Other definitions for adverse that I've seen before include "Hostile" , "Conflicting" , "Unfavourable - red vase (anag)" , "Dire" , "Unfavourable to evaders" .)