Transport military arm, thinking originally (4)
I believe the answer is:
raft
'transport' is the definition.
(rafting is a kind of transporting)
'military arm thinking originally' is the wordplay.
'military arm' becomes 'raf' (Royal Air Force).
'originally' indicates taking the first letters.
The first letter of 'thinking' is 't'.
'raf'+'t'='RAFT'
(Other definitions for raft that I've seen before include "Flat floating structure used as boat" , "Floating mass of logs" , "Wooden craft" , "vessel" , "Flat, floating platform" .)