Fruit gobbled up by little mongrel (5)
I believe the answer is:
lemon
'fruit' is the definition.
(lemon is a kind of fruit)
'up by little mongrel' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'by' is a charade indicator (letters next to each other).
'little' becomes 'l' (abbreviation for little).
'mongrel' becomes 'nome' (I can't justify this - if you can you should believe this answer much more).
'l' after 'nome' is 'nomel'.
'nomel' in reverse letter order is 'LEMON'.
'gobbled' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for lemon that I've seen before include "Shade of yellow - fruit" , "Fruit-tree" , "Defective object" , "Fruit; sort of grass, sole" , "Small evergreen fruit tree" .)