One tending to be less fat (6)
I believe the answer is:
leaner
'be less fat' is the definition.
('lean' can be the opposite of 'fat' so perhaps 'leaner' is reasonable for 'less fat')
'one tending' is the wordplay.
I cannot quite see how this works, but
'one' could be 'ane' (synonyms) and 'ane' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for leaner that I've seen before include "Thinner, with less fat" , "less productive" , "less bulky" , "less corpulent?" , "One with an inclination" .)