Explorer needs one in support (7)
I believe the answer is:
pioneer
'explorer' is the definition.
(thesaurus)
'one in support' is the wordplay.
'in' means one lot of letters goes inside another.
'support' becomes 'pier' (pier is a kind of support).
'one' inserted within 'pier' is 'PIONEER'.
'needs' is the link.
(Other definitions for pioneer that I've seen before include "Frontiersman" , "Launch" , "First to explore a new area" , "Open up an area" , "Lead the way" .)