Excel with soldiers in the open air (7)
I believe the answer is:
outdoor
'in the open air' is the definition.
(outdoor means out in the open air)
'excel with soldiers' is the wordplay.
'excel' becomes 'outdo' (I've seen this before).
'with' means one lot of letters go next to another.
'soldiers' becomes 'OR' (Other Ranks).
'outdo'+'or'='OUTDOOR'
(Other definitions for outdoor that I've seen before include "not under cover" , "Not done in the house" , "in the garden, say" , "Open-air" , "This sport is not under a roof" .)