Mythical creature I rammed at sea? (7)
I believe the answer is:
mermaid
'mythical creature' is the definition.
(I've seen this before)
'i rammed at sea?' is the wordplay.
'at sea?' is an anagram indicator (I've seen 'at sea' mean this).
'i'+'rammed'='irammed'
'irammed' anagrammed gives 'MERMAID'.
(Other definitions for mermaid that I've seen before include "London theatre?" , "siren" , "Woman with tail of fish" , "Fabulous woman" , "Mythical sea creature" .)