Pulse is fast - increased by 49! (6)
I believe the answer is:
lentil
'pulse' is the definition.
(type of pulse)
'fast increased by 49' is the wordplay.
I cannot really understand how this works, but
'fast' could be 'lent' (Lent is a period of fasting in Christianity) and 'lent' is present in the answer.
'49' could be 'il' (Roman numerals) and 'il' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'is' is the link.
Can you help me to learn more?
(Other definitions for lentil that I've seen before include "'Seed, pulse (6)'" , "Lintel (anag.)" , "Leguminous vegetable" , "Edible leguminous plant seed" , "Edible dried pulse" .)