Measure constant speed, essentially covering a length by metro transport (9)
I believe the answer is:
kilometre
'measure constant speed' is the definition.
'kilometre' can be an answer for 'measure' (I have seen 'Measure of distance' mean 'kilometre' so perhaps 'measure' could also mean 'kilometre'). I am unsure of the 'constant speed' bit.
'essentially covering a length by metro transport' is the wordplay.
'essentially' becomes 'kie' (I can't justify this - if you can you should give a lot more credence to this answer).
'covering' indicates putting letters inside.
'a length' becomes 'l'.
'by' means one lot of letters go next to another.
'transport' indicates an anagram (I've seen 'transported' mean this).
'l'+'metro'='lmetro'
'lmetro' is an anagram of 'lometr'.
'kie' placed around 'lometr' is 'KILOMETRE'.
Can you help me to learn more?
(Other definitions for kilometre that I've seen before include "1000m" , "5/8ths of a mile" , "K stands for this" , "0.621 of a mile" , "Measure of distance" .)