One inclined to become slimmer (6)
I believe the answer is:
leaner
'slimmer' is the definition.
('lean' can be similar in meaning to 'slim')
'one inclined' is the wordplay.
I cannot quite see how this works, but
'one' could be 'ane' (synonyms) and 'ane' is found within the answer.
This explanation may well be incorrect...
'to become' is the link.
Can you help me to learn more?
(Other definitions for leaner that I've seen before include "less productive" , "One with an inclination" , "less bulky" , "less corpulent?" , "Thinner, with less fat" .)