Lose temper when the French buy the drinks! (4,3)
I believe the answer is:
lash out
'lose temper' is the definition.
I can't judge whether this defines the answer.
'the french buy the drinks' is the wordplay.
'the french' becomes 'la' ('the' in French).
'buy the drinks' becomes 'shout' (I can't explain this - if you can you should give a lot more credence to this answer).
'la'+'shout'='LASH OUT'
'when' acts as a link.
Can you help me to learn more?
(Other definitions for lash out that I've seen before include "React violently" , "Attack with blows or words - using whip?" , "spend a fortune" , "fight vigorously" , "Vigorously attack" .)