They're oddly compact in size for transport hubs (7)
I believe the answer is:
termini
'transport hubs' is the definition.
(plural of terminus)
'they're oddly compact in size' is the wordplay.
'oddly' means one should take alternating letters (letters in odd-numbered positions).
'compact in size' becomes 'mini' (I can't explain this - if you can you should believe this answer much more).
The alternating letters of 'theyre' are 'ter'.
'ter'+'mini'='TERMINI'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for termini that I've seen before include "End stations" , "Interim (anag.)" , "End-points" , "Stops" , "Final parts" .)