One cadet in uniform's holding back (6)
I believe the answer is:
united
'one' is the definition.
(synonyms)
'cadet in uniform's holding back' is the wordplay.
'holding' indicates a hidden word.
'back' says the letters should be written backwards.
'detinu' is hidden in the letters of 'cadet in uniforms'.
'detinu' backwards is 'UNITED'.
(Other definitions for united that I've seen before include "in league" , "Joined into a single entity" , "... we stand, divided we fall" , "See 10" , "Of one mind" .)