Skinniest product of ten ales (7)
I believe the answer is:
leanest
'skinniest' is the definition.
('lean' can be similar in meaning to 'skinny')
'product of ten ales' is the wordplay.
'product of' is an anagram indicator.
'ten'+'ales'='tenales'
'tenales' with letters rearranged gives 'LEANEST'.
(Other definitions for leanest that I've seen before include "Most thin" , "Least fatty" , "Poorest-quality" , "Least fat" , "Most skinny" .)