Mister Lee Burn is mistaken (9)
I believe the answer is:
nebuliser
'mister' is the definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?
'lee burn is mistaken' is the wordplay.
'mistaken' is an anagram indicator.
'lee'+'burn'+'is'='leeburnis'
'leeburnis' with letters rearranged gives 'NEBULISER'.
Can you help me to learn more?
(Other definitions for nebuliser that I've seen before include "Used to inhale the drug" , "Drug delivery system" , "Device for producing a fine spray, for inhaling a medicinal drug, say" , "dispensed by this" , "Spray - blue rinse (anag)" .)