India's sect, one involved in religious festival (13)
I believe the answer is:
ascensiontide
'religious festival' is the definition.
(Christian festival)
'india's sect one involved' is the wordplay.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'indias'+'sect'+'one'='indiassectone'
'indiassectone' anagrammed gives 'ASCENSIONTIDE'.
'in' is the link.
(Other definitions for ascensiontide that I've seen before include "The ten days leading up to Whitsun Eve" , "Period before Whitsun" .)