Religious leader has time for Costello's partner (6)
I believe the answer is:
abbott
'costello's partner' is the definition.
(I've seen this in another clue)
'religious leader has time' is the wordplay.
'religious leader' becomes 'abbot' (head of an abbey).
'has' means one lot of letters go next to another.
'time' becomes 't' (abbreviation).
'abbot'+'t'='ABBOTT'
'for' acts as a link.
(Other definitions for abbott that I've seen before include "Costello's comedy partner" , "Costello's film partner" , "Bud ... and Lou Costello" , "Double act partner of Lou Costello" , "one of a comic duo" .)