Crossword Genius

Nuns depart after duffing up boxers, say? (10)

Ross

I believe the answer is:

underpants

I'm a little stuck... Click here to teach me more about this clue!

'up boxers say?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'nuns depart after duffing' is the wordplay.
'nuns depart' anagrams to 'UNDERPANTS'.
However, I'm not clear how this is indicated.

Can you help me to learn more?

(Other definitions for underpants that I've seen before include "cover and support" , "drawers" , "Shorts" , "Intimate clothing" , "Close-fitting garment" .)

I've seen this clue in The Telegraph.
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