Fifteen from London run without clothes (8)
I believe the answer is:
saracens
'fifteen from london' is the definition.
(Saracens rugby union team)
'run without clothes' is the wordplay.
'run' becomes 'race' ('race' can be a synonym of 'run').
'without' becomes 'sans' (I've seen this before**).
'clothes' means one lot of letters goes inside another.
'race' put inside 'sans' is 'SARACENS'.
(Other definitions for saracens that I've seen before include "They were opponents of the Crusaders" , "Medieval Crusaders' foes" , "enemies of Christians" .)