Like a cat going in to occupy shelter (7)
I believe the answer is:
leonine
'like a cat' is the definition.
(I've seen this before)
'going in to occupy shelter' is the wordplay.
'going' becomes 'on' (both can mean functioning or activated).
'to occupy' indicates putting letters inside (inserted letters occupy or enter the word).
'shelter' becomes 'lee' (as in lee side, the side sheltered from wind).
'on'+'in'='onin'
'onin' put into 'lee' is 'LEONINE'.
(Other definitions for leonine that I've seen before include "Relating to a carnivore or to certain popes" , "Papal" , "Like a big cat" , "like one with largest share" , "None lie (anag) - like a lion" .)