Permanently go back into splits (3,5)
I believe the answer is:
for keeps
'permanently' is the definition.
(I've seen this before)
'go back into splits' is the wordplay.
'go' becomes 'pee' (both can mean to urinate).
'back' shows that the letters should be reversed in order.
'into' is an insertion indicator.
'splits' becomes 'forks' (I can't explain this - if you can you should believe this answer much more).
'pee' in reverse letter order is 'eep'.
'eep' placed within 'forks' is 'FOR KEEPS'.
Can you help me to learn more?
(Other definitions for for keeps that I've seen before include "in favour of strongholds" , "indefinitely" , "with serious intent" , "Always" , "Permanently (colloq.)" .)