Friend, one of five picking up a final piece of ocean litter (9)
I believe the answer is:
palanquin
'litter' is the definition.
(litter carried as a form of transport)
'friend one of five picking up a final piece of ocean' is the wordplay.
'friend' becomes 'pal' (pal can mean a friend).
'one of five' becomes 'quin' (short for quintuplet, one of five children).
'picking up' indicates putting letters inside.
'final piece of' indicates one should take the final letters.
The final letter of 'ocean' is 'n'.
'pal'+'quin'='palquin'
'a'+'n'='an'
'palquin' placed around 'an' is 'PALANQUIN'.
(Other definitions for palanquin that I've seen before include "Box carried on poles" , "Shoulder-borne litter for one person" .)