Man with a message tries out short part of speech first (10)
I believe the answer is:
advertiser
'man with a message' is the definition.
(someone promoting a particular advertising message)
'tries out short part of speech first' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'short' means to remove the last letter.
'part of speech' becomes 'adverb' (adverb is a kind of part of speech).
'first' is a charade indicator (letters next to each other).
'adverb' with its final letter taken away is 'adver'.
'tries' anagrammed gives 'tiser'.
'tiser' after 'adver' is 'ADVERTISER'.
(Other definitions for advertiser that I've seen before include "He pushes" , "Promoter" , "commercial employee" , "One publicising a product (S, not Z)" , "who does the plugging" .)