Indian music when the jelly's served up (4)
I believe the answer is:
raga
'indian music' is the definition.
(I've seen this before)
'the jelly's served up' is the wordplay.
'the jelly' becomes 'agar' (gelatinous substance).
'served up' says the letters should be written in reverse.
'agar' in reverse letter order is 'RAGA'.
'when' is the link.
(Other definitions for raga that I've seen before include "Basis of improvisation in Indian music" , "Indian musical scale" , "Indian musical composition" , "Indian melody" , "Classical Indian piece" .)