Rearing dog on pastries and fish (7)

'fish' is the definition.
(guppy is a type of fish)

'rearing dog on pastries' is the wordplay.
'rearing' is a reversal indicator (as in a horse rearing up).
'dog' becomes 'pug' (pug is a kind of dog).
'on' says to put letters next to each other.
'pastries' becomes 'pies' (pie is a kind of pastry).
'pug' backwards is 'gup'.
'gup'+'pies'='GUPPIES'

'and' acts as a link.