Linear extent (9)
'linear extent' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for length that I've seen before include "Linear dimension" , "Measurement from end to end" , "Linear extent" , "Duration, extent" , "Duration or distance" .)
(Other definitions for dimension that I've seen before include "measure" , "Proportion" , "size" , "Length or breadth, say" .)