Speech, lines about a lexicon (10)
I believe the answer is:
dictionary
'lexicon' is the definition.
('dictionary' can be a synonym of 'lexicon')
'speech lines about a' is the wordplay.
'speech' becomes 'diction' (I've seen this in other clues).
'lines' becomes 'ry' (abbreviation for railway).
'about' is a charade indicator (letters next to each other).
'ry' put after 'a' is 'ary'.
'diction'+'ary'='DICTIONARY'
(Other definitions for dictionary that I've seen before include "reference work" , "Where to find the meaning of life, for example!" , "Work full of meaning" , "Brewer's creation, say" , "glossary" .)