River of froth? (5)
I believe the answer is:
forth
'river' is the definition.
(Scottish river)
'of froth?' is the wordplay.
'of' is an anagram indicator (the answer comes from or 'of' the letters).
'froth' is an anagram of 'FORTH'.
(Other definitions for forth that I've seen before include "Scottish river and firth" , "Onward; the Firth of - -" , "Out and away -- UK shipping forecast area" , "River and estuary in eastern Scotland" , "Water" .)