Bill to support popular former partner? That's rough (7)
I believe the answer is:
inexact
'rough' is the definition.
('inexact' can be similar in meaning to 'rough')
'bill to support popular former partner?' is the wordplay.
'bill' becomes 'act' (I've seen this in other clues**).
'to support' says to put letters next to each other (in a down clue, some letters hold others up).
'popular' becomes 'in'.
'former partner?' becomes 'ex'.
'in'+'ex'='inex'
'act' after 'inex' is 'INEXACT'.
'that's' acts as a link.
(Other definitions for inexact that I've seen before include "Vague" , "Not quite right" , "Lax" , "Fuzzy - untrue" , "Not precise, approximate" .)