Sell uniform seen in battle (7)
I believe the answer is:
auction
'sell' is the definition.
(auctioning is a kind of selling)
'uniform seen in battle' is the wordplay.
'uniform' becomes 'U' (phonetic alphabet: alpha, bravo, charlie etc.).
'seen in' means one lot of letters goes inside another.
'battle' becomes 'action' (military action).
'u' placed within 'action' is 'AUCTION'.
(Other definitions for auction that I've seen before include "Public sale to highest bidders" , "Caution about method of sale" , "Selling" , "Public sale - type of bridge" , "A lot will be seen in this" .)