Formal answer length most important (6)
I believe the answer is:
primal
'most important' is the definition.
'formal answer length' is the wordplay.
'formal' becomes 'prim' (I've seen this in other clues).
'answer' becomes 'a' (abbreviation eg in Q and A).
'length' becomes 'l'.
'prim'+'a'+'l'='PRIMAL'
(Other definitions for primal that I've seen before include "First; primitive" , "Primeval, original" , "Original; chief" , "Original, fundamental" , "Existing from the beginning, basic" .)