What could be Brighton line is pursuing speed (10)
I believe the answer is:
racecourse
'what could' is the definition.
I can't tell whether this defines the answer.
'brighton line is pursuing speed' is the wordplay.
'brighton line' becomes 'course' (I can't justify this - if you can you should give a lot more credence to this answer).
'is pursuing' says to put letters next to each other.
'speed' becomes 'race' (synonyms).
'course' after 'race' is 'RACECOURSE'.
'be' acts as a link.
Can you help me to learn more?
(Other definitions for racecourse that I've seen before include "Venue for meeting" , "Eg, Aintree" , "Where to find better" , "Doncaster, for example?" , "E.g. Ascot" .)