Supply stopping nearly at once, which is outstanding (9)
I believe the answer is:
prominent
'is outstanding' is the definition.
(synonyms)
'supply stopping nearly at once' is the wordplay.
'supply' becomes 'mine' (a mine is a supply of something).
'stopping' is an insertion indicator (stopping can mean blocking a gap).
'nearly' means to remove the last letter.
'at once' becomes 'pronto' (pronto can mean at once or quickly).
'pronto' with its last letter removed is 'pront'.
'mine' placed inside 'pront' is 'PROMINENT'.
'which' acts as a link.
(Other definitions for prominent that I've seen before include "Noticeable or notable" , "Important, famous" , "Standing out" , "Important; protuberant" , "Conspicuous or outstanding" .)