Diver at sea going round lake with Yankee showing mischief (7)
I believe the answer is:
devilry
'mischief' is the definition.
(I know that mischief can be written as devilry)
'diver at sea going round lake with yankee' is the wordplay.
'at sea' is an anagram indicator.
'going round' indicates putting letters inside (some letters go around others).
'lake' becomes 'L' (geographical abbreviation).
'with' means one lot of letters go next to another.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'diver' anagrammed gives 'devir'.
'devir' going around 'l' is 'devilr'.
'devilr'+'y'='DEVILRY'
'showing' is the link.
(Other definitions for devilry that I've seen before include "wickedness" , "Wicked activity" , "reckless mischief?" , "wicked stuff" , "It's sheer malice" .)