Military transport heading for Tobruk reversed track with it (9)
I believe the answer is:
troopship
'military transport' is the definition.
(ship for transporting troops)
'heading for tobruk reversed track with it' is the wordplay.
'heading for' suggests taking the first letters.
'reversed' says the letters should be written in reverse.
'track' becomes 'spoor' (track left by an animal).
'with it' becomes 'hip' (both can mean cool or fashionable).
The initial letter of 'tobruk' is 't'.
'spoor' backwards is 'roops'.
't'+'roops'+'hip'='TROOPSHIP'
(Other definitions for troopship that I've seen before include "Army vessel" , "As carrier" , "Military transport vessel" , "boat" .)