Second line written in pen predominantly (6)
I believe the answer is:
mostly
'predominantly' is the definition.
(thesaurus)
'second line written in pen' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'line' becomes 'l' (used when specifying particular lines from a poem).
'written in' means one lot of letters goes inside another.
'pen' becomes 'sty' (both can mean an animal enclosure).
'l' put within 'sty' is 'stly'.
'mo'+'stly'='MOSTLY'
(Other definitions for mostly that I've seen before include "For the greater part; usually" , "In large part" , "More often than not" , "Largely" , "On the whole" .)