It’s time for stag do on river (3)
I believe the answer is:
rut
'it's time for stag' is the definition.
I can't tell whether this defines the answer.
'do on river' is the wordplay.
'do' becomes 'ut' (both can mean the first note in a musical scale).
'on' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation).
'ut' put after 'r' is 'RUT'.
Can you help me to learn more?
(Other definitions for rut that I've seen before include "Furrow or groove as in track" , "Deep groove as in a lane" , "Grove you should not remain in" , "mate" , "Mating period for deer" .)