Pulse is fast - over one fifty (6)
I believe the answer is:
lentil
'pulse' is the definition.
(thesaurus)
'fast over one fifty' is the wordplay.
'fast' becomes 'lent' (I've seen this before).
'over' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'fifty' becomes 'l' (roman numeral).
'lent'+'i'+'l'='LENTIL'
'is' acts as a link.
(Other definitions for lentil that I've seen before include "Lintel (anag.)" , "Edible seed of bean plant" , "Pulse for soup etc" , "'Seed, pulse (6)'" , "Small edible seed" .)