Sea creature shows weak balance with oxygen originally depleted (8)
I believe the answer is:
porpoise
'sea creature' is the definition.
(porpoise is a type of sea creature)
'balance with oxygen originally depleted' is the wordplay.
'balance' becomes 'poise' (synonyms).
'with' means one lot of letters goes inside another.
'oxygen' becomes 'O' (atomic symbol for oxygen).
'originally depleted' becomes 'rp' (I can't justify this - if you can you should give a lot more credence to this answer).
'o'+'rp'='orp'
'poise' going around 'orp' is 'PORPOISE'.
'shows weak' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
(Other definitions for porpoise that I've seen before include "Marine mammal" , "Sea beast" , "Beakless whale" , "Member of dolphin family" , "Animal" .)