An animal notebook Jack’s lost (5)
I believe the answer is:
otter
'an animal' is the definition.
(thesaurus)
'notebook jack's lost' is the wordplay.
'notebook' becomes 'jotter' (jotter is a kind of notebook).
'jack' becomes 'j' (playing card abbreviation).
'lost' indicates named letters should be taken away.
'jotter' with 'j' taken out is 'OTTER'.
(Other definitions for otter that I've seen before include "Holt dweller" , "Aquatic carnivore" , "Semi-aquatic creature" , "furry creature" , "Freshwater mammal with dark brown fur" .)