Sweet thing in fluster over boy (8)
I believe the answer is:
flapjack
'sweet thing' is the definition.
(flapjacks are sweet food items)
'fluster over boy' is the wordplay.
'fluster' becomes 'flap' (in a flap or in a fluster can mean in a panic).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'boy' becomes 'jack' (boy's name).
'flap'+'jack'='FLAPJACK'
'in' acts as a link.
(Other definitions for flapjack that I've seen before include "Oatcake" , "Biscuit made with oats, syrup and butter" , "It's sweet" , "Biscuit made with rolled oats" , "Chewy oat biscuit" .)