Put a stop to aluminium boring (5)
I believe the answer is:
banal
'boring' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'put a stop to aluminium' is the wordplay.
'put a stop to' becomes 'ban' (I've seen this in another clue).
'aluminium' becomes 'al' (Al is the chemical symbol for aluminium).
'ban'+'al'='BANAL'
Can you help me to learn more?
(Other definitions for banal that I've seen before include "tired" , "Trite and insipid" , "Old hat" , "Obvious and boring" , "'Hackneyed, trite (5)'" .)