Cove in film alongside lake (5)
I believe the answer is:
inlet
'cove' is the definition.
('inlet' can be a synonym of 'cove')
'in film alongside lake' is the wordplay.
'film' becomes 'et'.
'alongside' means one lot of letters go next to another.
'lake' becomes 'l'.
'et' put after 'l' is 'let'.
'in'+'let'='INLET'
(Other definitions for inlet that I've seen before include "An entrance or opening" , "Sound" , "Gulf" , "Mouth of creek" , "Small arm of the sea" .)