In Scotland go south, crossing eastern river (6)
I believe the answer is:
ganges
'river' is the definition.
(Ganges is an example)
'in scotland go south crossing eastern' is the wordplay.
'in scotland go' becomes 'gang' (Scottish word for 'go').
'south' becomes 'S' (synonyms).
'crossing' indicates putting letters inside.
'eastern' becomes 'e'.
'gang'+'s'='gangs'
'gangs' going around 'e' is 'GANGES'.
(Other definitions for ganges that I've seen before include "River flowing into the Bay of Bengal" , "Hindus' most sacred river" , "Sacred river of N India" , "Sacred Indian river" , "Holy water" .)