In contact with a good person to secure win (7)
I believe the answer is:
against
'in contact with' is the definition.
(thesaurus)
'a good person to secure win' is the wordplay.
'good person' becomes 'st' (saint).
'to secure' indicates putting letters inside.
'win' becomes 'gain' ('gain' can be a synonym of 'win'**).
'a'+'st'='ast'
'ast' enclosing 'gain' is 'AGAINST'.
(Other definitions for against that I've seen before include "Con" , "Opposing" , "Facing" , "Not in favour of" , "Not supporting" .)