Long to board Greek island trains (7)
I believe the answer is:
coaches
'trains' is the definition.
(I've seen this before)
'long to board greek island' is the wordplay.
'long' becomes 'ache' (to long for something is to ache for it).
'to board' indicates putting letters inside (inserted letters come on board).
'greek island' becomes 'cos' (Cos is a Greek island).
'ache' going inside 'cos' is 'COACHES'.
(Other definitions for coaches that I've seen before include "Trainers for athletes or transport vehicles" , "Trainers or buses" , "Instructors in sports" , "cabins at the back" , "Trains" .)