Nonsense penned by left-leaning neanderthal, balderdash (4)
I believe the answer is:
blah
'nonsense' is the definition.
(I've seen this in another clue)
'penned by left-leaning neanderthal balderdash' is the wordplay.
'left-leaning' says the letters should be written backwards (letters written right to left).
'penned by' says the answer is hidden in the clue.
'halb' is hidden within 'neanderthal balderdash'.
'halb' reversed gives 'BLAH'.
(Other definitions for blah that I've seen before include "Insipid" , "Bombast" , "Pretentious words" , "Pretentious waffle" , "bunkum!" .)