Girl has the French rising temper (6)
I believe the answer is:
anneal
'temper' is the definition.
(synonyms)
'girl has the french rising' is the wordplay.
'girl' becomes 'anne' (I've seen this before).
'has' means one lot of letters go next to another.
'the french' becomes 'la'.
'rising' is a reversal indicator (going up in a down clue).
'la' back-to-front is 'al'.
'anne'+'al'='ANNEAL'
(Other definitions for anneal that I've seen before include "Harden by heating and cooling" , "Heat and cool (metal/glass) to remove stresses" , "Heat-treat (metal or glass)" , "Heat and cool slowly to toughen, metal or glass" , "Toughen by heating and slow cooling" .)