Gathering coral primarily, diver with a tank etc for descent (5)
I believe the answer is:
scuba
'tank etc for descent' is the definition.
(equipment for descending in the sea)
'gathering coral primarily diver with a' is the wordplay.
'gathering' means one lot of letters goes inside another.
'primarily' says to take the initial letters.
'diver' becomes 'sub' (submarine).
'with' means one lot of letters go next to another.
The initial letter of 'coral' is 'c'.
'c' going inside 'sub' is 'scub'.
'scub'+'a'='SCUBA'
(Other definitions for scuba that I've seen before include "Diving equipment" , "Apparatus for divers" , "Skin-diver's device" , "Device that lets divers breathe under water" , "Skin-diving apparatus" .)