Soldiers work in spinning class (6)
I believe the answer is:
troops
'soldiers' is the definition.
(groups of soldiers)
'work in spinning class' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'in' is an insertion indicator.
'spinning' says the letters should be written backwards.
'class' becomes 'sort' (synonyms).
'sort' backwards is 'tros'.
'op' put within 'tros' is 'TROOPS'.
(Other definitions for troops that I've seen before include "Groups of soldiers" , "Soldiers, armed forces" , "Military forces" , "Army" , "Cavalry" .)