Find fault with old Scot touring Kenya's borders (4,2)
I believe the answer is:
pick at
'find fault with' is the definition.
(synonyms)
'old scot touring kenya's borders' is the wordplay.
'old scot' becomes 'Pict' (Picts were an ancient people of Scotland).
'touring' means one lot of letters goes inside another (tour can mean to go around).
'borders' says to hollow out the word (remove centre letters) (only the edge or border letters remain).
'kenya' with its middle taken out is 'ka'.
'pict' enclosing 'ka' is 'PICK AT'.
(Other definitions for pick at that I've seen before include "Criticise pettily" , "Toy with" , "Nigglingly criticise" , "Find fault with" , "Carp about" .)