Join girl in lake (5)
I believe the answer is:
merge
'join' is the definition.
(I've seen this before)
'girl in lake' is the wordplay.
'girl' becomes 'g'.
'in' indicates putting letters inside.
'lake' becomes 'mere' (mere is a kind of lake).
'g' placed within 'mere' is 'MERGE'.
(Other definitions for merge that I've seen before include "Combine, blend" , "Make one" , "Become one" , "Fuse" , "Join into one" .)