Game in which Harry is superior to mother (5)
I believe the answer is:
halma
'game' is the definition.
(board game)
'harry is superior to mother' is the wordplay.
'harry' becomes 'hal' (Hal can be a nickname for Harry or Henry).
'is superior to' says to put letters next to each other (in a down clue, some letters go above others).
'mother' becomes 'ma' (Ma is another word for mother or mum).
'hal'+'ma'='HALMA'
'in which' is the link.
(Other definitions for halma that I've seen before include "Hopping board game" , "played on board" , "Old board game" , "Leaping board game" , "Board-game" .)