Thin mist affects the metalworker (8)
I believe the answer is:
tinsmith
'the metalworker' is the definition.
(tinsmith is a kind of metalworker)
'thin mist affects' is the wordplay.
'affects' indicates an anagram (I've seen 'affecting' mean this).
'thin'+'mist'='thinmist'
'thinmist' anagrammed gives 'TINSMITH'.
(Other definitions for tinsmith that I've seen before include "Metal worker" , "A craftsman" , "Possible forger" , "Maker of articles with metal, symbol Sn" , "Sn worker" .)