Right flanks closer to perimeters do well (7)
I believe the answer is:
prosper
'do well' is the definition.
(I've seen this before)
'right flanks closer to perimeters' is the wordplay.
'right' becomes 'proper' (similar in meaning).
'flanks' is an insertion indicator (flank can mean to go on either side of).
'closer to' suggests the final letters (the letter which ends or closes the word).
The last letter of 'perimeters' is 's'.
'proper' placed around 's' is 'PROSPER'.
(Other definitions for prosper that I've seen before include "Do well, be successful in life" , "like Merimee, nominally" , "Get rich" , "Make good" , "Succeed, flourish" .)