Better soldiers leaving in the open air (5)
I believe the answer is:
outdo
'better' is the definition.
(to outdo is to beat or do better than)
'soldiers leaving in the open air' is the wordplay.
'soldiers' becomes 'OR' (Other Ranks).
'leaving' suggests deleting specific letters.
'in the open air' becomes 'outdoor' (outdoor means out in the open air).
'outdoor' with 'or' taken out is 'OUTDO'.
(Other definitions for outdo that I've seen before include "Perform better than another" , "Beat" , "Surpass someone at something" , "Exceed" , "Perform better than (someone)" .)