Declares valuable items when crossing border, finally (7)
I believe the answer is:
asserts
'declares' is the definition.
(asserting is a kind of declaring)
'valuable items when crossing border finally' is the wordplay.
'valuable items' becomes 'assets' (an asset is something valuable).
'when crossing' means one lot of letters goes inside another.
'finally' suggests the final letters.
The last letter of 'border' is 'r'.
'assets' going around 'r' is 'ASSERTS'.
(Other definitions for asserts that I've seen before include "States firmly" , "States confidently and forcefully" , "Declares to be true" , "States categorically" .)