Sick, mostly after having fast pulse (6)
I believe the answer is:
lentil
'pulse' is the definition.
(thesaurus)
'sick mostly after having fast' is the wordplay.
'sick' becomes 'ill' ('ill' can be a synonym of 'sick').
'mostly' means to remove the last letter.
'after having' says to put letters next to each other.
'fast' becomes 'lent' (I've seen this before).
'ill' with its final letter taken off is 'il'.
'il' after 'lent' is 'LENTIL'.
(Other definitions for lentil that I've seen before include "Pulse used in soup" , "Edible dried pulse" , "Pulse used in cooking" , "Type of soup" , "Vegetarian food" .)