Number with clever sound (7)
I believe the answer is:
tenable
'sound' is the definition.
(I've seen this before)
'number with clever' is the wordplay.
'number' becomes 'ten' (ten is a kind of whole number).
'with' is a charade indicator (letters next to each other).
'clever' becomes 'able' (I've seen this before).
'ten'+'able'='TENABLE'
(Other definitions for tenable that I've seen before include "Able to be believed" , "okay as regards defence" , "Well-founded" , "Viable" , "Based on sound reasoning or evidence" .)