Joined cadet in uniform, partially retreating (6)
I believe the answer is:
united
'joined' is the definition.
(uniting is a kind of joining)
'cadet in uniform partially retreating' is the wordplay.
'partially' indicates the answer is hidden within the clue.
'retreating' says the letters should be written in reverse.
'detinu' can be found hidden inside 'cadet in uniform'.
'detinu' reversed gives 'UNITED'.
(Other definitions for united that I've seen before include "Manchester _____, current Premiership champions with..." , "Allied or combined" , "Of like mind" , "cooperative" , "Mark -, Russian-born US abstract painter" .)