Crossword Genius

Crawler when quiet (3)

Ross

I believe the answer is:

asp

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'crawler' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'when quiet' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'as'+'p'='ASP'

Can you help me to learn more?

(Other definitions for asp that I've seen before include "Cold-blooded killer" , "Egyptian cobra" , "Snake, killed Cleopatra" , "Joe Blake is Australian species" , "Snake seen in a spa, strangely" .)

I've seen this clue in The Mirror.
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