Marine mammal having power, initially, or composure (8)
I believe the answer is:
porpoise
'marine mammal' is the definition.
(I've seen this before)
'power initially or composure' is the wordplay.
'initially' suggests taking the first letters.
'composure' becomes 'poise' (poise is a kind of composure).
The first letter of 'power' is 'p'.
'p'+'or'+'poise'='PORPOISE'
'having' is the link.
(Other definitions for porpoise that I've seen before include "Marine mammal" , "Beakless whale" , "Member of dolphin family" , "Cetacean" , "Dolphin-like beast" .)