Pulse is one fifty, following fast time (6)
I believe the answer is:
lentil
'pulse' is the definition.
(thesaurus)
'one fifty following fast time' is the wordplay.
'one' becomes 'i' (Roman numeral).
'fifty' becomes 'l' (roman numeral).
'following' is a charade indicator (letters next to each other).
'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 "Leguminous vegetable" , "Vegetarian food" , "Edible seed of bean plant" , "Food stuff" , "Pulse for soup etc" .)