Game at home stadium beginner messes up (5)
I believe the answer is:
ruins
'messes up' is the definition.
(I've seen this in another clue)
'game at home stadium beginner' is the wordplay.
'game' becomes 'RU' (rugby union).
'at home' becomes 'in' ('I'm in' can mean 'I'm at home').
'beginner' indicates taking the first letters.
The initial letter of 'stadium' is 's'.
'ru'+'in'+'s'='RUINS'
(Other definitions for ruins that I've seen before include "Broken-down remnants" , "Rubble" , "Collapsed, disintegrated buildings" , "Spoils" , "Damaged remains (of building)" .)