Opening of Trump's border for those people (4)
I believe the answer is:
them
'those people' is the definition.
(I've seen this before)
'opening of trump's border' is the wordplay.
'opening of' suggests taking the first letters.
'border' becomes 'hem' (I've seen this before**).
The first letter of 'trumps' is 't'.
't'+'hem'='THEM'
'for' is the link.
(Other definitions for them that I've seen before include "Let -- eat cake" , "Perhaps, the Opposition" , "Those people (as object)" , "... in another case, they" , "people objectively" .)