Old queen with a finally foul temper (6)
I believe the answer is:
anneal
'temper' is the definition.
(I know that temper can be written as anneal)
'old queen with a finally foul' is the wordplay.
'old queen' becomes 'Anne' (Queen Anne, 18th-century queen).
'with' means one lot of letters go next to another.
'finally' indicates one should take the final letters.
The last letter of 'foul' is 'l'.
'anne'+'a'+'l'='ANNEAL'
(Other definitions for anneal that I've seen before include "Heat and cool slowly in order to toughen" , "Long angry speech" , "Toughen by a process of gradually heating and cooling" , "Harden by heating and cooling" , "Destress using heat" .)