Old duke in Scottish river now (5)
I believe the answer is:
today
'now' is the definition.
(I know that now can be written as today)
'old duke in scottish river' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'duke' becomes 'd' (abbreviation).
'in' is an insertion indicator.
'scottish river' becomes 'tay' (the River Tay in Scotland).
'o'+'d'='od'
'od' put within 'tay' is 'TODAY'.
(Other definitions for today that I've seen before include "one of our papers, once" , "Presently" , "At the present time, in general" , "Yesterday's tomorrow" , "The present time or age" .)