A crawler when quiet (3)
I believe the answer is:
asp
'a crawler' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the 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 "Snake seen in a spa, strangely" , "Cleopatra-killing snake" , "Snake found in spa" , "Joe Blake is Australian species" , "Snake, killed Cleopatra" .)