Anniversary party, third to be held in the gulf (8)
I believe the answer is:
birthday
'anniversary' is the definition.
(I know that birthday is a type of anniversary)
'third to be held in the gulf' is the wordplay.
'to be' is an anagram indicator.
'held in' means one lot of letters goes inside another.
'the gulf' becomes 'bay' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'third' with letters rearranged gives 'irthd'.
'irthd' placed within 'bay' is 'BIRTHDAY'.
'party' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for birthday that I've seen before include "It comes around once a year" , "When presents are expected" , "Nothing to wear?" , "Anniversary of nativity" , "when one was delivered" .)