Promoter backs it through unfavourable run (10)
I believe the answer is:
advertiser
'promoter' is the definition.
(I've seen this before)
'backs it through unfavourable run' is the wordplay.
'backs' shows that the letters should be reversed in order (I've seen 'back' mean this).
'through' means one lot of letters goes inside another (I've seen this in other clues).
'unfavourable' becomes 'adverse' ('adverse' can be similar in meaning to 'unfavourable').
'run' becomes 'r' (cricket abbreviation).
'it' back-to-front is 'ti'.
'ti' going into 'adverse' is 'advertise'.
'advertise'+'r'='ADVERTISER'
(Other definitions for advertiser that I've seen before include "Promoter" , "commercial employee" , "He pushes" , "One publicising a product (S, not Z)" , "One who can use multiple media" .)