Pulse fast - 49? (6)
I believe the answer is:
lentil
'pulse' is the definition.
(type of pulse)
'fast 49?' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'49?' becomes 'IL' (Roman numerals).
'lent'+'il'='LENTIL'
(Other definitions for lentil that I've seen before include "Pulse for soup etc" , "A legume, its seed" , "High-protein pulse" , "Pulse used in soup" , "'Seed, pulse (6)'" .)