Old traveller might spoil company sport! (5,4)
I believe the answer is:
marco polo
'old traveller' is the definition.
(13th-century explorer)
'spoil company sport' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'company' becomes 'co' (abbreviation).
'sport' becomes 'polo' (polo is a kind of sport).
'mar'+'co'+'polo'='MARCO POLO'
'might' acts as a link.
(Other definitions for marco polo that I've seen before include "sheep" , "traveller in East" , "Merchant of Venice" , "Great Venetian explorer of China and Asia in 14th century" , "Venetian explorer of China in 13th/14th centuries" .)