Fish cake on tray for starter (5)
I believe the answer is:
troll
'fish' is the definition.
(trolling is a type of fishing)
'cake on tray for starter' is the wordplay.
'cake' becomes 'roll' (roll is a type of cake eg Swiss roll).
'on' says to put letters next to each other.
'for starter' suggests taking the first letters (I've seen 'for starters' mean this).
The initial letter of 'tray' is 't'.
'roll' put after 't' is 'TROLL'.
(Other definitions for troll that I've seen before include "Fish with a moving bait" , "Grotesque dwarf" , "Folklore monster" , "supposedly lives in a cave" , "Internet provoker; ugly being" .)