Reserve’s virtually shelved, being not the quickest (8)
I believe the answer is:
subsonic
'not the quickest' is the definition.
(as opposed to supersonic)
'reserve's virtually shelved' is the wordplay.
'reserve's' becomes 'subs' (I've seen this in other clues).
'virtually' means to remove the last letter (all but the last letter).
'shelved' becomes 'onice' (both can mean postponed).
'onice' with its last letter taken away is 'onic'.
'subs'+'onic'='SUBSONIC'
'being' acts as a link.
(Other definitions for subsonic that I've seen before include "Less speedy than sound" , "Travelling at less than Mach 1" , "Such low rates for Jumbos" , "Of speed lower than that of sound" , "At a speed less than that of sound" .)