Tom back with soldiers in an instant (6)
I believe the answer is:
moment
'an instant' is the definition.
('moment' can be a synonym of 'instant')
'tom back with soldiers' is the wordplay.
'back' shows that the letters should be reversed in order.
'with' means one lot of letters goes inside another.
'soldiers' becomes 'men' (I've seen this before).
'tom' back-to-front is 'mot'.
'mot' placed around 'men' is 'MOMENT'.
'in' is the link.
(Other definitions for moment that I've seen before include "Jiffy" , "Import" , "Short time - importance" , "Instant - importance" , "Significance; minute" .)