In time, the Spanish slow down (5)
I believe the answer is:
delay
'slow down' is the definition.
(delaying is a kind of slowing down)
'in time the spanish' is the wordplay.
'in' is an insertion indicator.
'time' becomes 'day' (day is a kind of time**).
'the spanish' becomes 'el'.
'day' going around 'el' is 'DELAY'.
(Other definitions for delay that I've seen before include "Hinder" , "Time lapse" , "Hold back" , "Put off; pause" , "Linger - slow down - put off" .)