The one who deserted from the regiment on Tuesday? (3)
I believe the answer is:
rat
'the one who deserted from' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'regiment on tuesday?' is the wordplay.
'regiment' becomes 'RA' (Royal Artillery).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'tuesday?' becomes 't'.
'ra'+'t'='RAT'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for rat that I've seen before include "Betrayer" , "Treacherous rodent" , "one escaping doomed vessel?" , "Abandon" , "Domestic rodent" .)