Mammoth’s tooth traveller originally found by Welsh river (4)
I believe the answer is:
tusk
'mammoth's tooth' is the definition.
(large tooth of a mammoth)
'traveller originally found by welsh river' is the wordplay.
'originally' indicates taking the first letters.
'found by' means one lot of letters go next to another.
'welsh river' becomes 'usk' (I've seen this before).
The first letter of 'traveller' is 't'.
't'+'usk'='TUSK'
(Other definitions for tusk that I've seen before include "Long protruding tooth" , "Walrus horn" , "Walrus's long tooth" , "Jumbo tooth" , "Elongated tooth" .)