Fast, but not when in France! (4)
I believe the answer is:
lent
'fast' is the definition.
(I've seen this before)
'but not when in france' is the wordplay.
'but' becomes 'len' (I can't explain this - if you can you should give a lot more credence to this answer).
'not' is a deletion indicator.
'when' becomes 'e' (I've seen this in other clues).
'in france' becomes 'et' (I've seen this in other clues).
'et' with 'e' taken away is 't'.
'len'+'t'='LENT'
(Other definitions for lent that I've seen before include "Gave for a limited period" , "Forty days of penance" , "Christian penitential period" , "Advanced" , "From Ash Wednesday to Holy Saturday" .)