Try supporting American leader for once
I believe the answer is:
ago
'once' is the definition.
(I've seen this in another clue)
'try supporting american leader' is the wordplay.
'try' becomes 'go' (both can mean an attempt).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'leader' says to take the initial letters.
The first letter of 'american' is 'a'.
'go' put after 'a' is 'AGO'.
'for' is the link.
(Other definitions for ago that I've seen before include "Previously" , "Gone, past, since" , "In past times" , "Back (in time)" , "Time past" .)