Pencil marking circling a line rejected as nonsense (4)
I believe the answer is:
blah
'nonsense' is the definition.
(blah blah blah)
'pencil marking circling a line rejected' is the wordplay.
'pencil marking' becomes 'HB' (as in an HB pencil).
'circling' is an insertion indicator (circling can mean going around or containing).
'line' becomes 'l' (used when specifying particular lines from a poem).
'rejected' shows that the letters should be reversed in order.
'a'+'l'='al'
'hb' placed around 'al' is 'halb'.
'halb' backwards is 'BLAH'.
'as' acts as a link.
(Other definitions for blah that I've seen before include "Bombast" , "Dull" , "Pretentious nonsense" , "Stupid 20" , "Meaningless substitute for tedious words" .)