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 definition defines the answer.
'do on river' is the wordplay.
'do' becomes 'ut' (both can mean the first note in a musical scale).
'on' says to put letters next to each other.
'river' becomes 'r' (abbreviation for river).
'ut' 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" .)