Promote a greater number as well (11)
I believe the answer is:
furthermore
'as well' is the definition.
(I've seen this before)
'promote a greater number' is the wordplay.
'promote' becomes 'further' (furthering is a kind of promoting).
'a greater number' becomes 'more' (I've seen this in another clue).
'further'+'more'='FURTHERMORE'
(Other definitions for furthermore that I've seen before include "In addition, besides" , "as well" , "to boot" , "Also" .)