They're regularly on very small stations (7)
I believe the answer is:
termini
'stations' is the definition.
(terminus is a kind of station)
'they're regularly on very small' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'on' means one lot of letters go next to another.
'very small' becomes 'mini' (I've seen this in another clue).
The alternate letters of 'theyre' are 'ter'.
'ter'+'mini'='TERMINI'
(Other definitions for termini that I've seen before include "Ends, extremities" , "Rail stations mainly in city centres" , "Some like Victoria" , "goals" , "End-points" .)