Angel fish circling centre of mere (6)
I believe the answer is:
cherub
'angel' is the definition.
(cherub is a kind of angel)
'fish circling centre of mere' is the wordplay.
'fish' becomes 'chub' (chub is a kind of fish**).
'circling' is an insertion indicator (circling can mean going around or containing).
'centre of' says to take the centre.
The centre of 'mere' is 'er'.
'chub' placed around 'er' is 'CHERUB'.
(Other definitions for cherub that I've seen before include "Angelic infant" , "sweet infant" , "Angelic baby" , "A heavenly winged babe" , "innocent young thing" .)