Fast and almost unhealthy pulse (6)
I believe the answer is:
lentil
'pulse' is the definition.
(I've seen this before)
'fast and almost unhealthy' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'and' is a charade indicator (letters next to each other).
'almost' means to remove the last letter (most of the word but not all of it).
'unhealthy' becomes 'i'll' (associated in meaning).
'ill' with its last letter taken off is 'il'.
'lent'+'il'='LENTIL'
(Other definitions for lentil that I've seen before include "'Seed, pulse (6)'" , "Food stuff" , "Pulse used in cooking" , "Edible leguminous plant seed" , "Kind of bean plant used in soup" .)