Eggs, when split by line, part (4)
I believe the answer is:
role
'part' is the definition.
(eg a theatrical role)
'eggs when split by line' is the wordplay.
'eggs' becomes 'roe' (fish eggs).
'when split by' indicates putting letters inside (inserted letters split into the word).
'line' becomes 'l' (used when specifying particular lines from a poem).
'roe' placed around 'l' is 'ROLE'.
(Other definitions for role that I've seen before include "A part of lore" , "Part played (in life?)" , "Impersonation" , "Acting part" , "Part in play, film etc" .)