In haste pupils advance (4,2)
I believe the answer is:
step up
'advance' is the definition.
(thesaurus)
'in haste pupils' is the wordplay.
'in' indicates a hidden word.
'STEP-UP' is hidden in the letters of 'haste pupils'.
(Other definitions for step up that I've seen before include "Sudden increase (in rate, output, etc)" , "- Bezukhov (War and Peace)" , "Advancement (on a ladder or career)" , "Raise by stages" , "To be promoted" .)