Measure is about the right length (5)
I believe the answer is:
metre
'length' is the definition.
(I've seen this before)
'measure is about the right' is the wordplay.
'measure' becomes 'mete' (meting things out is measuring them out).
'is about' is an insertion indicator.
'the right' becomes 'r' (common abbreviation).
'mete' going around 'r' is 'METRE'.
(Other definitions for metre that I've seen before include "Poetic rhythm" , "Approximately 39 inches" , "three and a bit of a fourth" , "Beat pattern (mus.)" , "Unit of length in metric system" .)