Football people cross supporting United -- dad's mistake (4,3)
I believe the answer is:
faux pas
'mistake' is the definition.
(faux-pas is a kind of mistake)
'football people cross supporting united dad's' is the wordplay.
'football people' becomes 'fa' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'cross' becomes 'x' (X is cross-shaped).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'united' becomes 'u'.
'dad's' becomes 'pas' ('pa' can be a synonym of 'dad').
'x' after 'u' is 'ux'.
'fa'+'ux'+'pas'='FAUX-PAS'
Can you help me to learn more?
(Other definitions for faux pas that I've seen before include "Social indiscretion" , "Mistake" , "Embarrassing blunder" , "Social error" , "Socially tactless act" .)