Pen lines in the thing? Don't start dude! It's a lot (9)
I believe the answer is:
plenitude
'it's a lot' is the definition.
(thesaurus)
'pen lines in the thing? don't start dude' is the wordplay.
'lines' becomes 'l' (abbreviation as in lines of poetry).
'in the' means one lot of letters goes inside another.
'thing?' becomes 'it'.
'don't start' means to remove the first letter.
'pen' placed around 'l' is 'plen'.
'dude' with its first letter taken off is 'ude'.
'plen'+'it'+'ude'='PLENITUDE'
(Other definitions for plenitude that I've seen before include "entirety" , "Eden lit up (anag.)" , "Literary sort of abundance or fullness" .)