Birds and marine mammals circling bottomless bay (8)
I believe the answer is:
seagulls
'birds' is the definition.
(seagull is a kind of bird)
'marine mammals circling bottomless bay' is the wordplay.
'marine mammals' becomes 'seals' (aquatic animals).
'circling' is an insertion indicator (circling can mean going around or containing).
'bottomless' means to remove the last letter.
'bay' becomes 'gulf' (I've seen this before).
'gulf' with its final letter taken off is 'gul'.
'seals' going around 'gul' is 'SEAGULLS'.
'and' acts as a link.
(Another definition for seagulls that I've seen is " Aquatic birds".)