Evergreen creeper (3)
(Other definitions for ivy that I've seen before include "Parasitical plant, companion to holly" , "Hedera helix" , "Climbing plant, associated with holly" , "Evergreen creeping plant" , "Climbing shrub; f. name" .)
'evergreen creeper?' is the definition.
The answer and definition can be both animals as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
This is the entire clue.
Can you help me to learn more?
(Another definition for grass snake that I've seen is " legless wild thing".)