Assembled concerning distance (5)
I believe the answer is:
metre
'distance' is the definition.
(unit of distance)
'assembled concerning' is the wordplay.
'assembled' becomes 'met' ('meet' can be a synonym of 'assemble').
'concerning' becomes 're' (both can mean on a particular topic).
'met'+'re'='METRE'
(Other definitions for metre that I've seen before include "Yard is smaller than this" , "Moisture remover" , "Beat pattern (mus.)" , "1.094 yards" , "measure of length" .)