Prohibit a line that's mundane (5)
I believe the answer is:
banal
'mundane' is the definition.
(I've seen this in another clue)
'prohibit a line' is the wordplay.
'prohibit' becomes 'ban' (banning is a kind of prohibiting).
'line' becomes 'l' (used when specifying particular lines from a poem).
'ban'+'a'+'l'='BANAL'
'that's' is the link.
(Other definitions for banal that I've seen before include "Obvious and dull" , "Stock" , "flat" , "Tediously commonplace" , "Tediously unoriginal" .)