Number in debt - but promising (6)
I believe the answer is:
vowing
'promising' is the definition.
(I've seen this before)
'number in debt' is the wordplay.
'number' becomes 'v' (V is an example**).
'in debt' becomes 'owing' (I've seen this before).
'v'+'owing'='VOWING'
'but' acts as a link.
(Other definitions for vowing that I've seen before include "Making solemn pledges" , "Promising" .)