Number involved in agnosticism on the rise (4)
I believe the answer is:
song
'number' is the definition.
(thesaurus)
'involved in agnosticism on the rise' is the wordplay.
'involved in' says the answer is hidden in the clue.
'on the rise' shows that the letters should be reversed in order.
'gnos' is hidden in the letters of 'agnosticism'.
'gnos' reversed gives 'SONG'.
(Other definitions for song that I've seen before include "Piece of vocal music" , "Musical rendition" , "Words set to music" , "Vocal number" , "Ballad, aria" .)