Bit of a looker ate restaurant starter in mess (6)
I believe the answer is:
retina
'bit of a looker' is the definition.
(part of an eye, something which looks)
'ate restaurant starter in mess' is the wordplay.
'starter' indicates taking the first letters.
'mess' is an anagram indicator.
The initial letter of 'restaurant' is 'r'.
'ate'+'r'+'in'='aterin'
'aterin' with letters rearranged gives 'RETINA'.
(Other definitions for retina that I've seen before include "Bad tear in part of the eye" , "Sensitive layer in eye" , "Viewer's part" , "Retain with part of eye" , "Ocular membrane" .)