PC turns up, greeting northern sea creature (7)
I believe the answer is:
dolphin
'sea creature' is the definition.
(I've seen this before)
'pc turns up greeting northern' is the wordplay.
'pc' becomes 'plod' (informal term for a police officer).
'turns up' shows that the letters should be reversed in order.
'greeting' becomes 'hi' (hi is a kind of greeting).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'plod' reversed gives 'dolp'.
'dolp'+'hi'+'n'='DOLPHIN'
(Other definitions for dolphin that I've seen before include "member in school" , "Porpoise-like fish" , "Intelligent marine mammal" , "Grampus" , "Sociable sea mammal, sacred in ancient Greece" .)