Tag lines Lincoln introduced (5)
I believe the answer is:
label
'tag' is the definition.
('label' can be a synonym of 'tag')
'lines lincoln introduced' is the wordplay.
'lines' becomes 'll' (I've seen this in other clues**).
'lincoln' becomes 'abe'.
'introduced' means one lot of letters goes inside another.
'll' enclosing 'abe' is 'LABEL'.
(Other definitions for label that I've seen before include "Classifying name" , "Put a name to" , "Sticker" , "Tag or designation" , "Designate" .)