Mysterious aircraft maintains bearing over lake, detective reveals (7)
I believe the answer is:
unfolds
'reveals' is the definition.
(unfolding is a kind of revealing)
'mysterious aircraft maintains bearing over lake detective' is the wordplay.
'mysterious aircraft' becomes 'ufo'.
'maintains' means one lot of letters goes inside another.
'bearing' becomes 'n'.
'over lake' becomes 'l' (I am not sure about the 'over' bit.**).
'detective' becomes 'ds' (detective sergeant).
'ufo' going around 'n' is 'unfo'.
'unfo'+'l'+'ds'='UNFOLDS'
(Other definitions for unfolds that I've seen before include "(Of a story) develops" , "Opens, spreads out" , "Gradually reveals" .)