Pulse fast? I'll fail to finish? (6)
I believe the answer is:
lentil
'pulse' is the definition.
(type of pulse)
'fast? i'll fail to finish?' is the wordplay.
'fast?' becomes 'lent' (Lent is a period of fasting in Christianity).
'fail to finish?' means to remove the last letter (I've seen 'failing to finish' mean this).
'ill' with its final letter taken off is 'il'.
'lent'+'il'='LENTIL'
(Other definitions for lentil that I've seen before include "Kind of bean plant used in soup" , "Lintel (anag.)" , "High-protein pulse" , "Type of soup" , "Vegetarian food" .)