Diving acronym (5)
I believe the answer is:
scuba
'diving acronym' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for scuba that I've seen before include "Self-contained underwater breathing apparatus (acronym)" , "Device that lets divers breathe under water" , "Diving equipment" , "Skin-diver's device" , "Aqualung" .)