Mum's goal, at Twickenham, is good plain food (4)
I believe the answer is:
mash
'food' is the definition.
(mashed potato)
'mum's goal at twickenham is good plain' is the wordplay.
I cannot really see how this works, but
'mum's' could be 'mas' ('ma' can be a synonym of 'mum') and 'mas' is present in the answer.
A single letter 'h' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for mash that I've seen before include "Forces hospital" , "Animal feed" , "Crush; soft mixture" , "Ground potatoes" , "(Crush into) pulp" .)