Old city church favoured mischievous child (6)
I believe the answer is:
urchin
'mischievous child' is the definition.
(I've seen this before)
'old city church favoured' is the wordplay.
'old city' becomes 'ur' (ancient city state).
'church' becomes 'ch' (common abbreviation).
'favoured' becomes 'in' (I've seen this in other clues).
'ur'+'ch'+'in'='URCHIN'
(Other definitions for urchin that I've seen before include "seafood for some" , "Scruffy little kid" , "Hedgehog - ragamuffin" , "Mischievous child and hedgehog" , "Ragged child or sea creature with round shell" .)