Trooper runs after retreating gents bitten by croc (10)
I believe the answer is:
cavalryman
'trooper' is the definition.
('cavalryman' can be a synonym of 'trooper')
'runs after retreating gents bitten by croc' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'after' says to put letters next to each other.
'retreating gents' becomes 'val' (I can't explain this - if you can you should give a lot more credence to this answer).
'bitten by' is an insertion indicator (I've seen this in other clues).
'croc' becomes 'cayman' (type of crocodile).
'r' after 'val' is 'valr'.
'valr' put into 'cayman' is 'CAVALRYMAN'.
(Other definitions for cavalryman that I've seen before include "Mounted soldier" , "Horse soldier" .)