Assembled concerning length (5)
I believe the answer is:
metre
'length' is the definition.
(I've seen this before)
'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 "Approx. thirty nine point three seven inches" , "Beat pattern (mus.)" , "Unit of length in metric system" , "Rhythmic arrangement of syllables in verse" , "Poetic rhythm" .)