Fight naked in British lake (5)
I believe the answer is:
brawl
'fight' is the definition.
(I know that fight can be written as brawl)
'naked in british lake' is the wordplay.
'naked' becomes 'raw' (I've seen this before).
'in' means one lot of letters goes inside another.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'lake' becomes 'L' (geographical abbreviation).
'b'+'l'='bl'
'raw' going into 'bl' is 'BRAWL'.
(Other definitions for brawl that I've seen before include "Ruckus" , "Unruly fight" , "Public punch-up" , "Abounding" , "Affray" .)