A period of fasting among sources of fruit and sweeties (10)

'sweeties' is the definition.
(valentine is a kind of sweetie)

'a period of fasting among sources of fruit' is the wordplay.
'period of fasting' becomes 'Lent' (fast leading up to Easter).
'among' means one lot of letters goes inside another.
'sources of fruit' becomes 'vines' (eg grapevines).
'a'+'lent'='alent'
'alent' put inside 'vines' is 'VALENTINES'.

'and' is the link.