Warning a member about lake (5)
I believe the answer is:
alarm
'warning' is the definition.
(I've seen this before)
'a member about lake' is the wordplay.
'member' becomes 'arm' (arm is a kind of member).
'about' is a charade indicator (letters next to each other).
'lake' becomes 'l'.
'arm' put after 'l' is 'larm'.
'a'+'larm'='ALARM'
(Other definitions for alarm that I've seen before include "Frighten, disturb" , "A rousing sound in the morning" , "Safety bell or alert signal" , "Unnerve" , "Scare or worry" .)