Pulse is one fifty after fast time (6)
I believe the answer is:
lentil
'pulse' is the definition.
(thesaurus)
'one fifty after fast time' is the wordplay.
'one' becomes 'i' (Roman numeral).
'fifty' becomes 'l' (roman numeral).
'after' means one lot of letters go next to another.
'fast time' becomes 'lent' (I've seen this before).
'i'+'l'='il'
'il' after 'lent' is 'LENTIL'.
'is' acts as a link.
(Other definitions for lentil that I've seen before include "Pulse used in cooking" , "Kind of bean plant used in soup" , "bit of food" , "Small edible seed" , "Leguminous vegetable" .)