Fast, the Italian's pulse (6)
I believe the answer is:
lentil
'pulse' is the definition.
(type of pulse)
'fast the italian's' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'the italian' becomes 'il' (Italian for 'the').
'lent'+'il'='LENTIL'
(Other definitions for lentil that I've seen before include "Food stuff" , "High-protein pulse" , "Plant - pulse" , "bit of food" , "Small seed used for food" .)