Acceptable number to dial, perhaps (5)
I believe the answer is:
valid
'acceptable' is the definition.
(thesaurus)
'number to dial perhaps' is the wordplay.
'number' becomes 'v' (V is an example**).
'perhaps' indicates anagramming the letters.
'dial' with letters rearranged gives 'alid'.
'v'+'alid'='VALID'
(Other definitions for valid that I've seen before include "Legally in force" , "Legal, authentic" , "Based on reason or law" , "Well-founded" , "Legally good" .)