Flat in a revolting state during function (6)
I believe the answer is:
supine
'flat' is the definition.
(thesaurus)
'in a revolting state during function' is the wordplay.
'in a revolting state' becomes 'up'.
'during' indicates putting letters inside.
'function' becomes 'sine' (mathematical function).
'up' going into 'sine' is 'SUPINE'.
(Other definitions for supine that I've seen before include "Slow in acting" , "Laid-back" , "looking up" , "Use pin (anag.)" , "Prostrate" .)