Faslane's rapidly storing up weapons supplies (8)
I believe the answer is:
arsenals
'weapons supplies' is the definition.
(an arsenal is a supply of weapons)
'faslane's rapidly storing up' is the wordplay.
'storing' indicates the answer is hidden within the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'slanesra' is hidden within 'faslanes rapidly'.
'slanesra' backwards is 'ARSENALS'.
(Other definitions for arsenals that I've seen before include "Arms depots" , "Collections of arms and ammunition" .)