Gather hash in island heading back (8)
I believe the answer is:
assemble
'gather' is the definition.
('assemble' can be a synonym of 'gather')
'hash in island heading back' is the wordplay.
'hash' becomes 'mess' (I've seen this before).
'in' indicates putting letters inside.
'island' becomes 'elba' (I've seen this before**).
'heading back' shows that the letters should be reversed in order.
'mess' put inside 'elba' is 'elbmessa'.
'elbmessa' backwards is 'ASSEMBLE'.
(Other definitions for assemble that I've seen before include "Put together all the components" , "Gather or congregate together" , "Come or bring group together" , "Gather or collect together" , "See lambs (anag) - gather together" .)