Let wall be built around nothing (5)
I believe the answer is:
allow
'let' is the definition.
(synonyms)
'wall be built around nothing' is the wordplay.
'be built' is an anagram indicator.
'around' means one lot of letters goes inside another.
'nothing' becomes 'o' (looks like zero - 0).
'wall' with letters rearranged gives 'allw'.
'allw' placed around 'o' is 'ALLOW'.
(Other definitions for allow that I've seen before include "Admit as acceptable" , "Tolerate" , "OK" , "Grant" , "Permit, let happen" .)