Changed gear crossing border (4)
I believe the answer is:
edge
'border' is the definition.
(synonyms)
'changed gear crossing' is the wordplay.
'crossing' says the answer is hidden in the clue (the clue crosses over the answer).
'EDGE' can be found hidden inside 'changed gear'.
(Other definitions for edge that I've seen before include "Guitarist with U2" , "on which one is ill at ease?" , "Periphery" , "Border - meeting place" , "Keenness" .)