Permanently split, seep out (3,5)
I believe the answer is:
for keeps
'permanently' is the definition.
(I've seen this before)
'split seep out' is the wordplay.
'split' becomes 'fork' (I've seen this before).
'out' is an anagram indicator (out can mean wrong or inaccurate).
'seep' with letters rearranged gives 'eeps'.
'fork'+'eeps'='FOR KEEPS'
(Other definitions for for keeps that I've seen before include "Permanently (colloq.)" , "with serious intent" , "Always" , "indefinitely" , "in favour of strongholds" .)