Prohibit everything, almost, in flat (5)
I believe the answer is:
banal
'flat' is the definition.
(I've seen this before)
'prohibit everything almost' is the wordplay.
'prohibit' becomes 'ban' (banning is a kind of prohibiting).
'everything' becomes 'all' (all things).
'almost' means to remove the last letter (most of the word but not all of it).
'all' with its last letter taken off is 'al'.
'ban'+'al'='BANAL'
'in' acts as a link.
(Other definitions for banal that I've seen before include "Old hat" , "Boring or trivial" , "Lacking originality" , "Ordinary and uninteresting" , "Overfamiliar through overuse" .)