Marine creature with compact power or elegance (8)
I believe the answer is:
porpoise
'marine creature' is the definition.
'with compact power or elegance' is the wordplay.
'with' says to put letters next to each other.
'compact power' becomes 'p' (abbreviation or 'compact' version of power).
'elegance' becomes 'poise' (I've seen this before).
'p'+'or' is 'por'.
'por'+'poise'='PORPOISE'
(Other definitions for porpoise that I've seen before include "Swimmer" , "Sea beast" , "Beakless whale" , "Small aquatic mammal with blunt snout" , "Small whale with blunt snout" .)