Pulse is fast on crossword setter over fifty (6)
I believe the answer is:
lentil
'pulse' is the definition.
(type of pulse)
'fast on crossword setter over fifty' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'crossword setter' becomes 'I' ('I' referring to the person who set the crossword).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'fifty' becomes 'l' (Roman numeral).
'lent'+'i'+'l'='LENTIL'
'is' is the link.
(Other definitions for lentil that I've seen before include "Type of soup" , "Round flat seed" , "Small edible seed" , "Edible leguminous plant seed" , "A legume, its seed" .)