Sufficient promotion? Compare! (8)
I believe the answer is:
adequate
'sufficient' is the definition.
(similar in meaning)
'promotion? compare' is the wordplay.
'promotion?' becomes 'ad' (short for advert).
'compare' becomes 'equate' (synonyms).
'ad'+'equate'='ADEQUATE'
(Other definitions for adequate that I've seen before include "Fair to middling" , "Good enough, acceptable" , "Sufficient, good enough" , "Good or plentiful enough" , "Sufficient or all right" .)