Group of soldiers playing polo at battalion's base (7)
I believe the answer is:
platoon
'group of soldiers' is the definition.
(type of military subdivision)
'playing polo at battalion's base' is the wordplay.
'playing' indicates anagramming the letters.
'base' indicates one should take the final letters (the base of the word).
The last letter of 'battalion' is 'n'.
'polo'+'at'='poloat'
'poloat' is an anagram of 'platoo'.
'platoo'+'n'='PLATOON'
(Other definitions for platoon that I've seen before include "Subdivision of a company" , "Company's subdivision" , "Group of soldiers" , "Small military unit" , "detail" .)