A mistake by the soccer people last month (5)
I believe the answer is:
fault
'a mistake' is the definition.
('fault' can be a synonym of 'mistake')
'by the soccer people last month' is the wordplay.
'by' says to put letters next to each other.
'the soccer people' becomes 'fa' (I can't explain this - if you can you should give a lot more credence to this answer).
'last month' becomes 'ult' (short for 'last month' in correspondence).
'fa'+'ult' is 'FAULT'.
Can you help me to learn more?
(Other definitions for fault that I've seen before include "Blame" , "Defect; culpability" , "Sin, error" , "An incorrect serve in tennis" , "Shortcoming" .)