When a pin's dropped in, they slither about in their cage (4)
I believe the answer is:
asps
'they slither about in their cage' is the definition.
The definition suggests an adverb but the answer is not.
'when a pin's dropped in' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'dropped' indicates named letters should be taken away.
'pins' with 'in' taken away is 'ps'.
'as'+'ps'='ASPS'
Can you help me to learn more?
(Other definitions for asps that I've seen before include "Cleopatra's cobras" , "Cleopatra's snakes" , "Pass around the snakes" , "Poisonous creepers" , "Egyptian cobras" .)