Invade once arch rebuilt (8)
I believe the answer is:
encroach
'invade' is the definition.
(to encroach upon is to invade)
'once arch rebuilt' is the wordplay.
'rebuilt' indicates an anagram.
'once'+'arch'='oncearch'
'oncearch' with letters rearranged gives 'ENCROACH'.
(Other definitions for encroach that I've seen before include "Intrude on somebody's territory or time" , "Advance beyond proper limit" , "Make inroads" , "Intrude on another's property or rights" , "'Trespass, infringe (8)'" .)