Increases when the pet cat is about to be returned (5,2)
I believe the answer is:
steps up
'increases' is the definition.
(stepping up is a kind of increasing)
'the pet cat is about to be returned' is the wordplay.
'cat' becomes 'puss' (puss is a kind of cat).
'is about' means one lot of letters goes inside another.
'to be returned' says the letters should be written in reverse (I've seen 'is returned' mean this).
'pet' put into 'puss' is 'puspets'.
'puspets' back-to-front is 'STEPS UP'.
'when' is the link.
(Other definitions for steps up that I've seen before include "Approaches" , "Accelerates" .)