Seemingly endless (12)
(Other definitions for interminable that I've seen before include "Tedious" , "Without end" , "Long drawn-out" , "Seeming likely to go on for ever" , "Endless" .)
'seemingly endless' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for vast that I've seen before include "Unusually great" , "Astronomical" , "Huge, enormous" , "Boundless" , "Extremely large or extensive" .)