Veg out from diet? (7,2)
I believe the answer is:
lighten up
'veg out' is the definition.
Although both the answer and definition are verbs in their base form, I don't see how they can define each other.
'diet?' is the wordplay.
I cannot quite see how this works, but
'diet?' could be 'light' ('light' can be similar in meaning to 'diet') and 'light' is found within the answer.
This may be the basis of clue (or it may be nonsense).
'from' acts as a link.
Can you help me to learn more?
(Other definitions for lighten up that I've seen before include "Relax!" , "Chill" , "Unwind" , "Plug in the fire" .)