Back on diet? Stop that trifle! (3)
I believe the answer is:
toy
'trifle' is the definition.
(I've seen this before)
'back on diet? stop' is the wordplay.
'back on' suggests the final letters (the letter at the back).
'stop' becomes 'oy' (I can't justify this - if you can you should believe this answer much more).
The last letter of 'diet' is 't'.
't'+'oy'='TOY'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for toy that I've seen before include "Trifle with plaything?" , "Miniature (dog)" , "One of spelt out words" , "Teddy or Dolly" , "Small breed" .)