Rates: tiniest payment settled up (5)
I believe the answer is:
tempi
'rates' is the definition.
(tempo is a kind of rate)
'tiniest payment settled up' is the wordplay.
'tiniest payment' becomes 'ip' (1p, one penny**).
'settled' becomes 'met' (I've seen this in other clues).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'ip'+'met'='ipmet'
'ipmet' in reverse letter order is 'TEMPI'.
(Other definitions for tempi that I've seen before include "Speeds (music)" , "Times" , "Beats" , "Rates" , "Speed controls" .)