Nearly sick following fast pulse (6)
I believe the answer is:
lentil
'pulse' is the definition.
(type of pulse)
'nearly sick following fast' is the wordplay.
'nearly' means to remove the last letter.
'sick' becomes 'ill' (synonyms).
'following' is a charade indicator (letters next to each other).
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'ill' with its last letter taken away is 'il'.
'il' put after 'lent' is 'LENTIL'.
(Other definitions for lentil that I've seen before include "Round flat seed" , "Food stuff" , "Leguminous vegetable" , "Pulse used in cooking" , "bit of food" .)