She sits in the nest (3)
I believe the answer is:
hen
'she' is the definition.
Both the definition and answer are singular nouns.
Maybe you can see a link between them that I can't see?
'sits in the nest' is the wordplay.
'sits in' indicates a hidden word (I've seen 'sat in' mean this).
'HEN' is hidden within 'the nest'.
Can you help me to learn more?
(Other definitions for hen that I've seen before include "This party is for women only" , "Insufficient in amount" , "Clucking female" , "Laying bird" , "Source of eggs" .)