Old money flowed into football club (5)
I believe the answer is:
franc
'old money' is the definition.
(I've seen this before)
'flowed into football club' is the wordplay.
'flowed' becomes 'ran' ('run' can be a synonym of 'flow').
'into' indicates putting letters inside.
'football club' becomes 'FC'.
'ran' inserted into 'fc' is 'FRANC'.
(Other definitions for franc that I've seen before include "Nonsense (colloq.)" , "Ex-Gallic coin" , "Former European currency" , "Old French money" , "Former French currency" .)