Artist — I’d backed American line (6)
I believe the answer is:
radius
'line' is the definition.
(radius is a kind of line)
'artist i'd backed american' is the wordplay.
'artist' becomes 'ra' (Royal Academician, member of Royal Academy of Arts).
'backed' shows that the letters should be reversed in order.
'american' becomes 'us'.
'id' backwards is 'di'.
'ra'+'di'+'us'='RADIUS'
(Other definitions for radius that I've seen before include "from centre to boundary" , "Part of build-up of arms" , "Bone - half of diameter" , "Outer arm bone" , "Bone of forearm - distance from centre" .)