Work to keep little animal outside window (3-2)
I believe the answer is:
pop-up
'window' is the definition.
(type of window on a computer)
'work to keep little animal outside' is the wordplay.
I cannot quite understand how this works, but
'work' could be 'op' (common abbreviation for 'opus') and 'op' is found within the answer.
'animal' could be 'pup' (pup is a kind of animal) and 'pup' is located in the answer.
No letters remain.
This explanation may well be incorrect...
(Other definitions for pop-up that I've seen before include "(Of book) having rising illustrations" , "Children's book or shop that appears suddenly" , "Like many toasters or computer menus" , "annoying advert" , "Like many electric toasters" .)