Ferry soldiers back repeatedly (2-2)
I believe the answer is:
ro-ro
'ferry' is the definition.
(short for roll-on/roll-off)
'soldiers back repeatedly' is the wordplay.
'soldiers' becomes 'or' (Other Rank).
'back' is a reversal indicator.
'repeatedly' says to double the letters.
'or' written backwards gives 'ro'.
'ro' duplicated is 'RO-RO'.
(Other definitions for ro-ro that I've seen before include "Transport for vehicles with drivers" , "Kind of ferry" , "Class of ferry/cargo ship" , "Type of ferry transport for vehicles" .)