Extra doctor back is what attracts interest (9)
I believe the answer is:
overdraft
'what attracts interest' is the definition.
'extra doctor back' is the wordplay.
'extra' becomes 'over'.
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'back' becomes 'aft' ('aft' is associated in meaning with 'back'**).
'over'+'dr'+'aft'='OVERDRAFT'
'is' is the link.
(Other definitions for overdraft that I've seen before include "Bank borrowing" , "Red balance" , "having bad position at bank" , "Bank account deficit" , "figure in red?" .)