Overrun southwest branch (5)
I believe the answer is:
swarm
'overrun' is the definition.
(thesaurus)
'southwest branch' is the wordplay.
'southwest' becomes 'SW' (synonyms).
'branch' becomes 'arm' (synonyms).
'sw'+'arm'='SWARM'
(Other definitions for swarm that I've seen before include "Move in large numbers like locusts" , "Large cloud (of insects)" , "Group of social insects" , "Army" , "many insects" .)