What a relatively light diet will get you? (7)
I believe the answer is:
slimmer
'what a relatively light' is the definition.
I can't judge whether this definition defines the answer.
'diet will get you?' is the wordplay.
'diet' becomes 'slim' (both can mean to attempt to lose weight).
'will get' is a charade indicator (letters next to each other).
'you?' becomes 'mer' (I can't justify this - if you can you should believe this answer much more).
'slim'+'mer'='SLIMMER'
Can you help me to learn more?
(Other definitions for slimmer that I've seen before include "in better shape?" , "More slenderly built" , "One attempting to lose weight" , "thinner" , "One losing weight" .)