Ace crossing lake and medium whirlpool (9)
I believe the answer is:
maelstrom
'whirlpool' is the definition.
(I know that whirlpool can be written as maelstrom)
'ace crossing lake and medium' is the wordplay.
'ace' becomes 'maestro' (both can mean an expert or virtuoso).
'crossing' means one lot of letters goes inside another.
'lake' becomes 'L' (geographical abbreviation).
'and' is a charade indicator (letters next to each other).
'medium' becomes 'm' (as in UK clothing sizes).
'maestro' going around 'l' is 'maelstro'.
'maelstro'+'m'='MAELSTROM'
(Other definitions for maelstrom that I've seen before include "Eddy" , "Strong circular current of water" , "violent upheaval" , "danger to shipping" , "Powerful whirlpool" .)