Fast time, but not for Pierre! (4)
I believe the answer is:
lent
'fast time' is the definition.
(I've seen this in another clue)
'not for pierre' is the wordplay.
'not' becomes 'n't' (synonyms).
'for' is a charade indicator (letters next to each other) (I've seen this in other clues).
'pierre' becomes 'le' (I can't explain this - if you can you should believe this answer much more).
'nt' put after 'le' is 'LENT'.
'but' is the link.
Can you help me to learn more?
(Other definitions for lent that I've seen before include "Pre-Easter period" , "Gave the use of for a time" , "Advanced" , "days before Easter" , "Time of fasting" .)