To some extent, they feel slippery (4)
I believe the answer is:
eels
'to some extent they feel slippery' is the definition.
The definition suggests an adverb but the answer is not.
'to some extent they feel slippery' is the wordplay.
'to some extent' indicates the answer is hidden within the clue (I've seen 'extent of' mean this (I've seen this in other clues)).
'EELS' is hidden in the letters of 'they feel slippery'.
Can you help me to learn more?
(Other definitions for eels that I've seen before include "Elongated fishes" , "Jellied --, traditional dish" , "(Slippery) fish" , "Long thin fish" , "Food for Cockney" .)