Make metal stiff (6)
I believe the answer is:
formal
'stiff' is the definition.
(similar in meaning)
'make metal' is the wordplay.
'make' becomes 'form' (synonyms).
'metal' becomes 'al' (chemical symbol for aluminium).
'form'+'al'='FORMAL'
(Other definitions for formal that I've seen before include "Punctilious" , "Ceremonious or prescribed" , "stuffy" , "conventionally correct" , "Ceremonial; ordered" .)