Demolished one pub as required by legal instruction (8)
I believe the answer is:
subpoena
'legal instruction' is the definition.
(a subpoena is a court summons)
'demolished one pub as required' is the wordplay.
'demolished' indicates an anagram.
'required' says to put letters next to each other.
'one'+'pub'+'as'='onepubas'
'onepubas' with letters rearranged gives 'SUBPOENA'.
'by' acts as a link.
(Other definitions for subpoena that I've seen before include "Writ commanding person to appear in court" , "Writ compelling attendance in court" , ""Under penalty" (Latin)" , "Writ commanding court attendance" , "calls for appearance in court" .)