Move in large numbers, having suitable backing (4)
I believe the answer is:
teem
'move in large numbers' is the definition.
(I've seen this before)
'suitable backing' is the wordplay.
'suitable' becomes 'meet' ('meet' can be a synonym of 'suitable').
'backing' is a reversal indicator.
'meet' in reverse letter order is 'TEEM'.
'having' acts as a link.
(Other definitions for teem that I've seen before include "Come down hard" , "pelt" , "Flow copiously" , "Bear" , "numerous" .)