Bob and Carol crossing small mountain (8)
I believe the answer is:
shilling
'bob' is the definition.
(I've seen this before)
'carol crossing small mountain' is the wordplay.
'carol' becomes 'sing' (carol can mean to sing carols).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'small mountain' becomes 'hill' (I've seen this in another clue).
'sing' placed around 'hill' is 'SHILLING'.
'and' is the link.
(Other definitions for shilling that I've seen before include "Until 1971, ready here" , "Pre-decimalisation coin" , "soldier once took it from the King" , "Old unit of imperial money, worth 12 pence" , "Old piece" .)