Marginal former area between right and left (7)
I believe the answer is:
lateral
'marginal' is the definition.
Both the answer and definition are adjectives. Perhaps there's a link between them I don't understand?
'former area between right and left' is the wordplay.
'former' becomes 'late' (late can mean former or previous).
'area' becomes 'a' (maths abbreviation).
'between' is an insertion indicator.
'right and left' becomes 'rl' (I can't justify this - if you can you should give a lot more credence to this answer).
'a' placed within 'rl' is 'ral'.
'late'+'ral'='LATERAL'
Can you help me to learn more?
(Other definitions for lateral that I've seen before include "Way of thinking" , "This kind of sideways thinking does not use conventional logic" , "to one side" , "Possibly right" , "Sort of thinking" .)