Intervene when Kitty's upbringing is not tops (4,2)
I believe the answer is:
step in
'intervene' is the definition.
(I know that intervene can be written as step in)
'kitty's upbringing is not tops' is the wordplay.
'kitty's' becomes 'pets' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'upbringing' says the letters should be written in reverse.
'tops' indicates taking the first letters.
The first letters of 'is not' is 'in'.
'pets' reversed gives 'step'.
'step'+'in'='STEP-IN'
'when' is the link.
Can you help me to learn more?
(Other definitions for step in that I've seen before include "Substitute" , "enter" , "Type of garment" , "Interrupt" , "Shoe with no fastenings" .)