Platoons, say, some of best in USA, retreating (5)
I believe the answer is:
units
'platoons' is the definition.
(I've seen this in another clue)
'some of best in usa retreating' is the wordplay.
'some of' indicates the answer is hidden within the clue.
'retreating' is a reversal indicator.
'stinu' is hidden within 'best in usa'.
'stinu' back-to-front is 'UNITS'.
'say' acts as a link.
(Other definitions for units that I've seen before include "Detachments" , "Individual things or groups" , "military teams" , "Modules" , "Individual things as parts of a whole" .)