To retain order, attack vigorously (4,4)
I believe the answer is:
tear into
'attack vigorously' is the definition.
(I've seen this in another clue)
'to retain order' is the wordplay.
'order' indicates an anagram.
'to'+'retain'='toretain'
'toretain' is an anagram of 'TEAR INTO'.
(Other definitions for tear into that I've seen before include "Attack verbally" , "Scold furiously" , "handbag" .)