Pulse fast, nearly sick (6)
I believe the answer is:
lentil
'pulse' is the definition.
(thesaurus)
'fast nearly sick' is the wordplay.
'fast' becomes 'lent' (I've seen this before).
'nearly' means to remove the last letter.
'sick' becomes 'ill' ('ill' can be a synonym of 'sick').
'ill' with its final letter taken off is 'il'.
'lent'+'il'='LENTIL'
(Other definitions for lentil that I've seen before include "Pulse for soup etc" , "Pulse used in soup" , "Leguminous vegetable" , "High-protein pulse" , "Round flat seed" .)