Unit gathered near shelled area (5)
I believe the answer is:
metre
'unit' is the definition.
(unit of distance)
'gathered near shelled area' is the wordplay.
'gathered' becomes 'met' ('meet' can be a synonym of 'gather').
'near' says to put letters next to each other.
'shelled' indicates the central letters (the outer letters or shell are removed).
The centre of 'area' is 're'.
'met'+'re'='METRE'
(Other definitions for metre that I've seen before include "Approximately 39 inches" , "Approx. thirty nine point three seven inches" , "three and a bit of a fourth" , "Yard is smaller than this" , "Distance" .)