Endlessly going off on tirade is not clever (8)
I believe the answer is:
ignorant
'not clever' is the definition.
'endlessly going off on tirade' is the wordplay.
'endlessly' means to remove the last letter.
'off' indicates anagramming the letters.
'on' says to put letters next to each other.
'tirade' becomes 'rant' (I've seen this before).
'going' with its last letter taken off is 'goin'.
'goin' is an anagram of 'igno'.
'igno'+'rant'='IGNORANT'
'is' acts as a link.
(Other definitions for ignorant that I've seen before include "No rating (anag) - clueless" , "Empty-headed" , "Knowing nothing" , "Unlearned, lacking knowledge" , "Lacking knowledge or skill" .)