Trains regularly covered by ticket sellers for holidaymakers (8)
I believe the answer is:
tourists
'for holidaymakers' is the definition.
('tourist' can be a synonym of 'holidaymaker')
'trains regularly covered by ticket sellers' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'covered by' is an insertion indicator.
'ticket sellers' becomes 'touts' (as in ticket touts).
The alternate letters of 'trains' are 'ris'.
'ris' placed within 'touts' is 'TOURISTS'.
(Other definitions for tourists that I've seen before include "They travel for pleasure" , "Group of visitors" , "Visitors and sightseers" , "Holiday-makers from abroad" .)