Very fine inhaler I misused (8)
I believe the answer is:
hairline
'very fine' is the definition.
(I've seen this before)
'inhaler i misused' is the wordplay.
'misused' indicates an anagram.
'inhaler'+'i'='inhaleri'
'inhaleri' with letters rearranged gives 'HAIRLINE'.
(Other definitions for hairline that I've seen before include "Very narrow opening" , "area on the outskirts of Barnet?" , "Of a crack, very thin" , "Type of crack" , "(Very fine) crack or fracture" .)