Not merely a king penguin (7)
I believe the answer is:
emperor
'penguin' is the definition.
(emperor penguin is a kind of penguin)
'not merely a king' is the wordplay.
I cannot really see how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is present in the answer.
'king' could be 'r' (abbreviation of Latin rex) and 'r' is found in the answer.
The remaining letters 'em' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for emperor that I've seen before include "Monarch" , "Nero or penguin" , "Say, Nero" , "Pre-Rome (anag.)" , "Character in 24 ac" .)