Surrounded by a thousand in support (4)
I believe the answer is:
amid
'surrounded by' is the definition.
(amid means among or surrounded by)
'a thousand in support' is the wordplay.
'a thousand' becomes 'M' (Roman numeral for a thousand).
'in' indicates putting letters inside.
'support' becomes 'aid' (to aid is to help or support).
'm' going into 'aid' is 'AMID'.
(Other definitions for amid that I've seen before include "A dim sort of preposition" , "In the course of" , "Surrounded by" , "Amongst" , "In the middle of" .)