Closely allied (3-2-3)
I believe the answer is:
arm-in-arm
'closely allied' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for arm-in-arm that I've seen before include "Closely interlinked" , "Of people, linked together in friendly way" , "With limbs interlinked" , "Aggressive male" , "Closely linked" .)