Traveller's instrument on journey shows highest possible mph (5,2,5)
I believe the answer is:
speed of light
'highest possible mph' is the definition.
The definition and answer can be both related to time as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'traveller's instrument on journey' is the wordplay.
'traveller's instrument' becomes 'speedo' (I can't explain this - if you can you should believe this answer much more).
'on' means one lot of letters go next to another.
'journey' becomes 'flight' (flight is a kind of journey).
'speedo'+'flight'='SPEED OF LIGHT'
'shows' is the link.
Can you help me to learn more?
(Other definitions for speed of light that I've seen before include "The c in Einstein's equation" , "299,792,458 meters per second" , "The c in E=mc 2" , "299,792,458 metres per second" .)