Acceptable number to dial, perhaps (5)
I believe the answer is:
valid
'acceptable' is the definition.
(I've seen this before)
'number to dial perhaps' is the wordplay.
'number' becomes 'v' (V is an example).
'perhaps' indicates an anagram.
'dial' anagrammed 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" .)