Black Prince retrospective? Dull pretentious nonsense (4)
I believe the answer is:
blah
'dull pretentious nonsense' is the definition.
'black prince retrospective?' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'prince' becomes 'hal' (I've seen this before).
'retrospective?' says the letters should be written backwards (I've seen 'retrospective' mean this).
'hal' written backwards gives 'lah'.
'b'+'lah'='BLAH'
(Other definitions for blah that I've seen before include "Meaningless substitute for tedious words" , "bunkum!" , "Stupid 20" , "Bombast" , "Insipid" .)