They don't permit a number to see around (6)
I believe the answer is:
vetoes
'they don't permit a' is the definition.
The definition suggests an adverb but the answer is not.
'number to see around' is the wordplay.
'number' becomes 'v' (V is an example).
'around' indicates an anagram.
'to'+'see'='tosee'
'tosee' with letters rearranged gives 'etoes'.
'v'+'etoes'='VETOES'
Can you help me to learn more?
(Other definitions for vetoes that I've seen before include "Casts over-riding negative vote" , "Votes to block a decision" , "E votes to block decisions" , "Refuses to allow" .)