Transport arm of the military on top of tactics (4)
I believe the answer is:
raft
'transport' is the definition.
(I know that raft is a more specific form of the action transport)
'arm of the military on top of tactics' is the wordplay.
'arm of the military' becomes 'raf' (Royal Air Force).
'on top of' indicates taking the first letters.
The initial letter of 'tactics' is 't'.
'raf'+'t'='RAFT'
(Other definitions for raft that I've seen before include "Makeshift boat" , "Structure that floats" , "Flat structure for travel over water" , "Simple boat" , "water transport" .)