Emperor penguins ultimately eaten by sailor (4)
I believe the answer is:
tsar
'emperor' is the definition.
(I've seen this before)
'penguins ultimately eaten by sailor' is the wordplay.
'ultimately' indicates one should take the final letters.
'eaten by' is an insertion indicator.
'sailor' becomes 'tar' (slang term for a sailor).
The final letter of 'penguins' is 's'.
's' put within 'tar' is 'TSAR'.
(Other definitions for tsar that I've seen before include "Russian leader" , "Monarch of old Russia" , "Powerful man" , "Peter the Great, for example" , "Male monarch" .)