Surrounded by many in support (4)
I believe the answer is:
amid
'surrounded by' is the definition.
(amid means among or surrounded by)
'many in support' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'in' is an insertion indicator.
'support' becomes 'aid' (to aid is to help or support).
'm' going within 'aid' is 'AMID'.
(Other definitions for amid that I've seen before include "Surrounded by" , "In the centre of" , "In the course of" , "In the middle of" , "A dim sort of preposition" .)