Carry on as salesman, coming back hard (9)
I believe the answer is:
persevere
'carry on' is the definition.
(I've seen this before)
'salesman coming back hard' is the wordplay.
'salesman' becomes 'rep' (sales representative).
'coming back' shows that the letters should be reversed in order.
'hard' becomes 'severe' (synonyms).
'rep' reversed gives 'per'.
'per'+'severe'='PERSEVERE'
'as' acts as a link.
(Other definitions for persevere that I've seen before include "Keep going" , "Be persistent, refuse to stop" , "Remain" , "Struggle on!" , "Continue regardless" .)