Outspoken, expert about lake (5)
I believe the answer is:
bluff
'outspoken' is the definition.
('bluff' can be a synonym of 'outspoken')
'expert about lake' is the wordplay.
'expert' becomes 'buff' (I've seen this before).
'about' means one lot of letters goes inside another.
'lake' becomes 'l'.
'buff' placed around 'l' is 'BLUFF'.
(Other definitions for bluff that I've seen before include "Outspoken and hearty" , "Honest" , "Attempt to deceive" , "Steep bank; pretend confidence" , "Hoodwink" .)