Guarantee place advantage (6)
I believe the answer is:
pledge
'guarantee' is the definition.
(I've seen this in another clue)
'place advantage' is the wordplay.
'place' becomes 'pl' (abbreviation used in postal addresses).
'advantage' becomes 'edge' (having the edge is having an advantage).
'pl'+'edge'='PLEDGE'
(Other definitions for pledge that I've seen before include "Promise - security" , "Promise, oath" , "Solemn undertaking" , "Deposit" , "Firm commitment" .)