Large stream of volunteers on the third of May (3)
I believe the answer is:
tay
'large stream' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'volunteers on the third of may' is the wordplay.
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'the third of may' becomes 'y' (3rd letter of 'may').
'ta'+'y'='TAY'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for tay that I've seen before include "Scottish river in Katayama?" , "Longest Scottish river" , "The - Bridge Disaster (McGonagall)" , "Scotland's longest river" , "Scottish river and firth" .)