Feline one working - not so much outside (7)
I believe the answer is:
lioness
'feline' is the definition.
(I know that lioness is a type of Panthera leo)
'one working not so much outside' is the wordplay.
'one' becomes 'I' (Roman numeral).
'working' becomes 'on' (on can mean working or activated).
'not so much' becomes 'less' (I've seen this in another clue).
'outside' means one lot of letters goes inside another.
'i'+'on'='ion'
'ion' inserted inside 'less' is 'LIONESS'.
(Other definitions for lioness that I've seen before include "Queen of beasts?" , "Brave woman" , "Big beast's mate" , "Fierce female feline" , "Female pride member" .)