Don't allow that almost all are lacking in originality (5)
I believe the answer is:
banal
'are lacking in originality' is the definition.
The answer and definition are not the same part of speech. However, adjectives and past participle verbs occasionally define each other.
'don't allow that almost all' is the wordplay.
'don't allow' becomes 'ban' (I've seen this in other clues).
'that almost' means to remove the last letter (most of the word but not all of it).
'all' with its final letter taken off is 'al'.
'ban'+'al'='BANAL'
Can you help me to learn more?
(Other definitions for banal that I've seen before include "Pedestrian" , "Lacking originality" , "Obvious and dull" , "Boring or trivial" , "Mundane" .)