Interrupt idiots holding exercise class back (4,2)
I believe the answer is:
step in
'interrupt' is the definition.
(I've seen this before)
'idiots holding exercise class back' is the wordplay.
'idiots' becomes 'nits' (nit can mean a stupid person).
'holding' is an insertion indicator.
'exercise class' becomes 'pe' (physical education).
'back' says the letters should be written backwards.
'nits' placed around 'pe' is 'nipets'.
'nipets' in reverse letter order is 'STEP-IN'.
(Other definitions for step in that I've seen before include "Shoe with no fastenings" , "Substitute" , "Interrupt" , "Intervene dramatically, or at a senior level" , "enter" .)