Diving gear stowed aboard, initially worn by youngster (5)
I believe the answer is:
scuba
'diving gear' is the definition.
'stowed aboard initially worn by youngster' is the wordplay.
'initially' says to take the initial letters.
'worn by' is an insertion indicator (I've seen this in other clues).
'youngster' becomes 'cub' (I've seen this in other clues**).
The first letters of 'stowed aboard' is 'sa'.
'sa' placed around 'cub' is 'SCUBA'.
(Other definitions for scuba that I've seen before include "Sort of diver" , "Device that lets divers breathe under water" , "Aqualung" , "Skin-diver's device" , "Apparatus for divers" .)