Suffers hard in test for promotion (8)
I believe the answer is:
mailshot
'promotion' is the definition.
(promotional literature)
'suffers hard in test' is the wordplay.
'suffers' becomes 'ails' (ailing is a kind of suffering).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'in' is an insertion indicator.
'test' becomes 'MOT' (as in an MOT test for a vehicle).
'ails'+'h'='ailsh'
'ailsh' inserted inside 'mot' is 'MAILSHOT'.
'for' acts as a link.
(Other definitions for mailshot that I've seen before include "commercial literature" , "Leaflet sent to a large group at the same time" , "Promotional items widely dispatched" , "Communication" , "Postal item, often junk" .)