Gather expert containing mounting disorder... (8)
I believe the answer is:
assemble
'gather' is the definition.
(I know that gather can be written as assemble)
'expert containing mounting disorder' is the wordplay.
'expert' becomes 'able' (able can mean expert or talented).
'containing' is an insertion indicator.
'mounting' says the letters should be written backwards (in a down clue, letters mount or go up).
'disorder' becomes 'mess' (messing is a kind of disordering).
'mess' backwards is 'ssem'.
'able' enclosing 'ssem' is 'ASSEMBLE'.
(Other definitions for assemble that I've seen before include "Collect in one place" , "See lambs (anag) - gather together" , "Bring/come together" , "fit together" , "Get together" .)