Cult ran on grounds at night (9)
I believe the answer is:
nocturnal
'at night' is the definition.
(I've seen this before)
'cult ran on grounds' is the wordplay.
'grounds' indicates anagramming the letters (I've seen 'ground' mean this ('grind' the letters into a new form)).
'cult'+'ran'+'on'='cultranon'
'cultranon' is an anagram of 'NOCTURNAL'.
(Other definitions for nocturnal that I've seen before include "happening at night" , "Like a piece by Chopin?" , "Like owls and badgers" , "Relating to the night" , "Active after dark" .)