Ignore loch's odd rule after swimming (4-8)
I believe the answer is:
cold-shoulder
'ignore' is the definition.
('cold-shoulder' can be a synonym of 'ignore')
'loch's odd rule after swimming' is the subsidiary indication.
'after' means one lot of letters go next to another.
'swimming' indicates anagramming the letters.
'lochs'+'odd'+'rule'='lochsoddrule'
'lochsoddrule' is an anagram of 'coldshoulder'.
(Other definitions for cold-shoulder that I've seen before include "Brush off" , "Give (someone) a rebuff" , "Snub" , "Treat with indifference" , "Cut, ignore" .)