Some British submarines returning to spring a trap (6)
I believe the answer is:
ambush
'spring a trap' is the definition.
(I've seen this before)
'some british submarines returning' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'returning' says the letters should be written backwards.
'hsubma' can be found hidden inside 'british submarines'.
'hsubma' written backwards gives 'AMBUSH'.
'to' acts as a link.
(Other definitions for ambush that I've seen before include "Weakening" , "Lie in wait; trap" , "Lie in wait and attack" , "Lying in wait to attack" , "Attack from concealed position" .)