Fighting unit of soldiers retreating into cover? (5)
I believe the answer is:
troop
'fighting unit' is the definition.
(troop is a kind of military unit)
'soldiers retreating into cover?' is the wordplay.
'soldiers' becomes 'or' (Other Rank).
'retreating' says the letters should be written backwards.
'into' is an insertion indicator.
'cover?' becomes 'top' (topping is a kind of covering**).
'or' in reverse letter order is 'ro'.
'ro' going into 'top' is 'TROOP'.
'of' is the link.
(Other definitions for troop that I've seen before include "musical group perhaps" , "A company, unit" , "Military unit" , "Porto (anag.)" , "Flock, large group" .)