Mock group running football at city university (5)
I believe the answer is:
false
'mock' is the definition.
(thesaurus)
'group running football at city university' is the wordplay.
'group running football' becomes 'FA' (Football Association).
'at' means one lot of letters go next to another.
'city university' becomes 'LSE' (London School of Economics).
'fa'+'lse'='FALSE'
(Other definitions for false that I've seen before include "Erroneous" , "Artificial" , "Sham" , "Untrue - treacherous - counterfeit" , "Wrong" .)