Under top of glass eye, a long wiggly line (9)
I believe the answer is:
genealogy
'line' is the definition.
(thesaurus)
'under top of glass eye a long wiggly' is the wordplay.
'under' is a charade indicator (letters next to each other).
'top of' indicates taking the first letters.
'wiggly' is an anagram indicator.
The initial letter of 'glass' is 'g'.
'eye'+'a'+'long'='eyealong'
'eyealong' with letters rearranged gives 'enealogy'.
'g' next to 'enealogy' is 'GENEALOGY'.
(Other definitions for genealogy that I've seen before include "The study of ancestry" , "Study of lines of descent" , "Study of family descent" , "Study of family trees" , "Pedigree" .)