Let old wall be built round (5)
I believe the answer is:
allow
'let' is the definition.
(I know that let can be written as allow)
'old wall be built round' is the wordplay.
'old' becomes 'o' (abbreviation).
'be built' indicates anagramming the letters.
'round' indicates putting letters inside.
'wall' with letters rearranged gives 'allw'.
'o' placed into 'allw' is 'ALLOW'.
(Other definitions for allow that I've seen before include "Give leave to" , "Grant" , "praise no more" , "Sanction" , "Permit, consent to" .)