Tin makes alloy wrong (8)
I believe the answer is:
mistaken
'wrong' is the definition.
(similar in meaning)
'tin makes alloy' is the wordplay.
'alloy' indicates anagramming the letters.
'tin'+'makes'='tinmakes'
'tinmakes' with letters rearranged gives 'MISTAKEN'.
(Other definitions for mistaken that I've seen before include "Wrong in opinion, judgment etc." , "Wrong (in judgement)" , "Wrong-headed" , "'Wrong, misguided (8)'" , "Wrongly understood" .)