A number competent? That’s credible (7)
I believe the answer is:
tenable
'credible' is the definition.
(thesaurus)
'a number competent?' is the wordplay.
'a number' becomes 'ten' (ten is a kind of number**).
'competent?' becomes 'able' ('able' can be similar in meaning to 'competent').
'ten'+'able'='TENABLE'
'that's' is the link.
(Other definitions for tenable that I've seen before include "Viable" , "Able to be believed" , "possible to maintain" , "okay as regards defence" , "Of argument, rational and defensible" .)