Back rank chess piece is on (7)
I believe the answer is:
working
'on' is the definition.
'back rank chess piece' is the wordplay.
'back' says the letters should be written backwards.
'rank' becomes 'row' (**).
'chess piece' becomes 'king' (king is a kind of chess piece).
'row' backwards is 'wor'.
'wor'+'king'='WORKING'
'is' is the link.
(Other definitions for working that I've seen before include "Operating" , "Employed" .)