Airborne soldier gets a knock, coming back (4)
I believe the answer is:
para
'airborne soldier' is the definition.
(paratrooper)
'a knock coming back' is the wordplay.
'knock' becomes 'rap' (synonyms).
'coming back' says the letters should be written backwards.
'a'+'rap'='arap'
'arap' backwards is 'PARA'.
'gets' is the link.
(Other definitions for para that I've seen before include "A block of text; an airborne soldier (abbrs.)" , "Airdropped soldier" , "Soldier who's dropped in? (abbr)" , "Air-drop soldier" , "Soldier dropping to earth" .)