Pulse fast, extremely ill (6)
I believe the answer is:
lentil
'pulse' is the definition.
(type of pulse)
'fast extremely ill' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'ill' with its middle removed is 'il'.
'lent'+'il'='LENTIL'
(Other definitions for lentil that I've seen before include "Leguminous vegetable" , "Pulse used in soup" , "Plant - pulse" , "Pulse for soup etc" , "Edible seed of bean plant" .)