Gulf fish doctor's speciality gains song (11)
I believe the answer is:
carpentaria
'gulf' is the definition.
(I know that Carpentaria is a gulf)
'fish doctor's speciality gains song' is the wordplay.
I cannot really understand how this works, but
'fish' could be 'carp' (carp is a kind of fish) and 'carp' is found within the answer.
'song' could be 'aria' (piece of music in an opera) and 'aria' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for carpentaria that I've seen before include "Australian gulf" , "Gulf of - is off north Australia" .)