Group beginning to bathe in lake (5)
I believe the answer is:
combo
'group' is the definition.
(a combo can mean a musical group)
'beginning to bathe in lake' is the wordplay.
'beginning to' says to take the initial letters.
'in' means one lot of letters goes inside another.
'lake' becomes 'como' (I have seen 'Italian lake ' mean 'como' so perhaps 'lake' could also mean 'como').
The initial letter of 'bathe' is 'b'.
'b' put inside 'como' is 'COMBO'.
(Other definitions for combo that I've seen before include "Small jazz or dance band" , "musicians" , "group of players" , "Small jazz group" , "Small pop group" .)