Retreat harder on sons failing to clinch victory (4,2,4,5)
I believe the answer is:
draw in ones horns
'retreat' is the definition.
(I've seen this in another clue)
'harder on sons failing to clinch victory' is the wordplay.
'failing' indicates anagramming the letters.
'to clinch' indicates putting letters inside.
'victory' becomes 'win' (a win is a victory).
'harder'+'on'+'sons'='harderonsons'
'harderonsons' with letters rearranged gives 'draoneshorns'.
'draoneshorns' going around 'win' is 'DRAW IN ONES HORNS'.
(Other definitions for draw in ones horns that I've seen before include "save" , "finish self-portrait as a bit of a devil?" , "Economise" , "but complete diabolical self-portrait?" , "Scale down ambition" .)