Clever article in the main (6)
I believe the answer is:
brainy
'clever' is the definition.
(I've seen this before)
'article in the main' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'in the' is an insertion indicator.
'main' becomes 'briny' (synonyms).
'a' going inside 'briny' is 'BRAINY'.
(Other definitions for brainy that I've seen before include "Highly intelligent" , "smart?" , "Bright" , "Clever, got lots of grey matter" , "Showing intelligence" .)