Cove in film about lake (5)
I believe the answer is:
inlet
'cove' is the definition.
(synonyms)
'in film about lake' is the wordplay.
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'about' is a charade indicator (letters next to each other).
'lake' becomes 'l' (geographical abbreviation).
'et' put after 'l' is 'let'.
'in'+'let'='INLET'
(Other definitions for inlet that I've seen before include "Small arm of the sea" , "Fiord" , "access point" , "Arm off a larger body of water between headlands" , "An entrance or opening" .)