One on a diet may turn out so (7)
I believe the answer is:
slimmer
'may turn out so' is the definition.
I can't tell whether this definition defines the answer.
'one on a diet' is the wordplay.
'one' becomes 'mer' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' means one lot of letters go next to another.
'a diet' becomes 'slim' (both can mean to attempt to lose weight).
'mer' put after 'slim' is 'SLIMMER'.
Can you help me to learn more?
(Other definitions for slimmer that I've seen before include "thinner" , "One losing weight" , "More slender" , "More slenderly built" , "with less fat" .)