Fast or mostly slow? (4)
I believe the answer is:
lent
'fast' is the definition.
(I've seen this before)
'mostly slow?' is the wordplay.
'mostly' means to remove the last letter.
'slow?' becomes 'lento' ('lento' can be similar in meaning to 'slow').
'lento' with its last letter taken off is 'LENT'.
'or' is the link.
(Other definitions for lent that I've seen before include "From Ash Wednesday to Holy Saturday" , "Gave for a limited period" , "Gave the use of for a time" , "Forty days of penance" , "Period from Ash Wednesday to Easter" .)