Go over Black Mountains (6)
I believe the answer is:
bridge
'go over' is the definition.
(thesaurus)
'black mountains' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'mountains' becomes 'ridge' (a ridge is a type of mountain range).
'b'+'ridge'='BRIDGE'
(Other definitions for bridge that I've seen before include "Where four hands are needed" , "Connection crossing river etc" , "Card-game" , "Viaduct" , "''Like a ... over troubled water'' (Simon and Garfunkel)" .)