Mischief an ensign has at sea? (11)
I believe the answer is:
shenanigans
'mischief' is the definition.
(thesaurus)
'an ensign has at sea?' is the wordplay.
'at sea?' indicates an anagram (I've seen 'at sea' mean this).
'an'+'ensign'+'has'='anensignhas'
'anensignhas' is an anagram of 'SHENANIGANS'.
(Other definitions for shenanigans that I've seen before include "Mischief; dishonest activity" , "dirty work!" , "Monkey business" , "Mischievous trickery" .)