Seal fate: explorer soon goes travelling east (4,4,5)
I believe the answer is:
cook ones goose
'seal fate' is the definition.
(to cook one's goose is to seal one's fate)
'explorer soon goes travelling east' is the wordplay.
'explorer' becomes 'Cook' (18th-century explorer Captain James Cook).
'travelling' indicates an anagram.
'east' becomes 'e' (synonyms).
'soon'+'goes'='soongoes'
'soongoes' anagrammed gives 'onesgoos'.
'cook'+'onesgoos'+'e'='COOK ONES GOOSE'
(Other definitions for cook ones goose that I've seen before include "Spoil a person's plans" , "Scupper plans" .)