Sweetheart proud about fast time with sweetheart (9)
I believe the answer is:
valentine
'sweetheart' is the definition.
(valentine is a kind of sweetheart)
'proud about fast time with sweetheart' is the wordplay.
'proud' becomes 'vain' ('vain' can be similar in meaning to 'proud'**).
'about' indicates putting letters inside.
'fast time' becomes 'lent' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'vain' going around 'lent' is 'valentin'.
'valentin'+'e'='VALENTINE'
(Other definitions for valentine that I've seen before include "Love card" , "Gentle Veronese" , "Potential lover" , "Message for February 14" , "One of Shak.'s Two Gentlemen" .)