Trap cluster of French submarines in retreat (6)
I believe the answer is:
ambush
'trap' is the definition.
('ambush' can be a synonym of 'trap')
'cluster of french submarines in retreat' is the wordplay.
'cluster of' indicates the answer is hidden within the clue.
'in retreat' shows that the letters should be reversed in order.
'hsubma' can be found hidden inside 'french submarines'.
'hsubma' written backwards gives 'AMBUSH'.
(Other definitions for ambush that I've seen before include "Lie in wait and attack" , "Attack from concealment" , "Attack from concealed position" , "Sudden attack" , "Jump" .)