Mashed praties can be bad at sea (7)
I believe the answer is:
pirates
'can be bad at sea' is the definition.
I can't judge whether this definition defines the answer.
'mashed praties' is the wordplay.
'mashed' is an anagram indicator.
'praties' with letters rearranged gives 'PIRATES'.
Can you help me to learn more?
(Other definitions for pirates that I've seen before include "Robbers at sea" , "felons found in craft?" , "Robbers on the high seas" , "Buccaneers" , "More than one brigand" .)