Gather disorder splits island where Boney was held up, ... (8)
I believe the answer is:
assemble
'gather' is the definition.
('assemble' can be a synonym of 'gather')
'disorder splits island where boney was held up' is the wordplay.
'disorder' becomes 'mess' (mess is a kind of disorder**).
'splits' indicates putting letters inside.
'island where boney was held' becomes 'elba' (Napoleon Bonaparte was imprisoned on Elba).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'mess' inserted inside 'elba' is 'elbmessa'.
'elbmessa' back-to-front is 'ASSEMBLE'.
(Other definitions for assemble that I've seen before include "Build" , "See lambs (anag) - gather together" , "Put together all the components" , "Get together" , "Gather together at meeting place" .)