Being serious, set out to support merit (7)
I believe the answer is:
earnest
'being serious' is the definition.
('earnest' can be similar in meaning to 'serious')
'set out to support merit' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'to support' says to put letters next to each other (in a down clue, some letters hold others up).
'merit' becomes 'earn' (I've seen this before).
'set' anagrammed gives 'est'.
'est' put after 'earn' is 'EARNEST'.
(Other definitions for earnest that I've seen before include "Serious; down-payment" , "Resolute" , "Pledge - far from frivolous" , "guarantee" , "Seriously eager" .)