Span top of British mountain chain (6)
I believe the answer is:
bridge
'span' is the definition.
(I know that span can be written as bridge)
'top of british mountain chain' is the wordplay.
'top of' says to take the initial letters.
'mountain chain' becomes 'ridge' (I've seen this in another clue).
The first letter of 'british' is 'b'.
'b'+'ridge'='BRIDGE'
(Other definitions for bridge that I've seen before include "Where four hands are needed" , "pontoon, perhaps" , "Card game - pontoon - couple" , "Partial denture; span" , "Game of cards" .)