Fast nothing - it's slow (5)
I believe the answer is:
lento
'slow' is the definition.
(similar in meaning)
'fast? nothing' is the wordplay.
'fast?' becomes 'lent' (Lent is a period of fasting in Christianity).
'nothing' becomes 'O' (looks like zero - 0).
'lent'+'o'='LENTO'
'it's' acts as a link.
(Other definitions for lento that I've seen before include "It's slower than adagio" , "Musically low speed" , "Scored slowly" , "Instruction to players" , "Slow (music)" .)