Very thin ale sent for replacement (7)
I believe the answer is:
leanest
'very thin' is the definition.
'ale sent for replacement' is the wordplay.
'for replacement' is an anagram indicator (I've seen this in other clues).
'ale'+'sent'='alesent'
'alesent' is an anagram of 'LEANEST'.
(Other definitions for leanest that I've seen before include "Most thin" , "Least fatty" , "Most unproductive" , "most meagre type" , "Most skinny" .)