The pair who shook Cleo up (6)
I believe the answer is:
couple
'the pair' is the definition.
('couple' can be a synonym of 'pair')
'shook cleo up' is the wordplay.
'shook' is an anagram indicator.
'cleo'+'up'='cleoup'
'cleoup' with letters rearranged gives 'COUPLE'.
'who' acts as a link.
(Other definitions for couple that I've seen before include "Duo, brace" , "Bonnie and Clyde, say" , "Pair - put together" , "item" , "The pair to join together" .)