Seek tall eccentric, very thin (8)
I believe the answer is:
skeletal
'very thin' is the definition.
(extremely thin or emaciated)
'seek tall eccentric' is the wordplay.
'eccentric' indicates anagramming the letters.
'seek'+'tall'='seektall'
'seektall' is an anagram of 'SKELETAL'.
(Other definitions for skeletal that I've seen before include "Comprising just the bare bones" , "pinched" , "Consisting of bones" , "reduced to a minimum" , "Like a framework of lakelets" .)