Very thin leek, last to be cooked (8)
I believe the answer is:
skeletal
'very thin' is the definition.
(I've seen this before)
'leek last to be cooked' is the wordplay.
'to be cooked' is an anagram indicator.
'leek'+'last'='leeklast'
'leeklast' is an anagram of 'SKELETAL'.
(Other definitions for skeletal that I've seen before include "wasted" , "Bony, gaunt" , "pinched" , "Dreadfully thin" , "Like a framework of lakelets" .)