Most slender (8)
'most slender' is the definition.
('thin' can be a synonym of 'slender')
This is the entire clue.
(Other definitions for thinnest that I've seen before include "Most fine" , "Superlatively poor" , "Most slim" , "Most slender as in tenths" , "Narrowest or leanest" .)
'most slender' is the definition.
('lean' can be similar in meaning to 'slender')
This is the entire clue.
(Other definitions for leanest that I've seen before include "most meagre type" , "Least fat" , "Most thin" , "Most skinny" , "Poorest-quality" .)