So you'll find half of them over the United States (4)
I believe the answer is:
thus
'so' is the definition.
(I know that so can be written as thus)
'half of them over the united states' is the wordplay.
'half of' suggests halving the letters.
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'the united states' becomes 'U.S.' (abbreviation for the United States).
'them' cut in half is 'th'.
'th'+'us'='THUS'
'you'll find' acts as a link.
(Other definitions for thus that I've seen before include "Consequently" , "In this way" , "Like so" , "Shut (anag.)" , "In this manner" .)