Cat left to face one working (4)
I believe the answer is:
lion
'cat' is the definition.
(I know that lion is a type of cat)
'left to face one working' is the wordplay.
'to face' indicates taking the first letters.
'one' becomes 'i' (Roman numeral).
'working' becomes 'on' (on can mean working or activated).
The initial letter of 'left' is 'l'.
'l'+'i'+'on'='LION'
(Other definitions for lion that I've seen before include "one showing unusual courage?" , "Leo" , "Cat, the king of the beasts" , "Carnivore" , "Big cat with pride" .)