Beat king with small military units (5)
I believe the answer is:
tanks
'military units' is the definition.
(military vehicles)
'beat king with small' is the wordplay.
'beat' becomes 'tan' (I've seen this before).
'king' becomes 'k' (abbreviation for king in chess).
'with' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'tan'+'k'+'s'='TANKS'
(Other definitions for tanks that I've seen before include "Bombs" , "Reservoirs - military vehicles" , "Armed vehicles" , "Armored military vehicles" , "Armoured military vehicles" .)