Deny no-one outside access (6)
I believe the answer is:
negate
'deny' is the definition.
(thesaurus)
'no-one outside access' is the wordplay.
'no one' becomes 'ne' (I can't justify this - if you can you should give a lot more credence to this answer).
'outside' says to put letters next to each other (outside can mean adjacent to).
'access' becomes 'gate' (I've seen this in other clues).
'ne'+'gate'='NEGATE'
Can you help me to learn more?
(Other definitions for negate that I've seen before include "Age ten (anag)" , "Show to be false" , "Make invalid" , "Opera by Puccini" , "Cause to have no effect" .)