The person responsible for upsetting our cart (7)
I believe the answer is:
curator
'the person' is the definition.
(curator is a kind of person)
'upsetting our cart' is the wordplay.
'upsetting' indicates an anagram.
'our'+'cart'='ourcart'
'ourcart' with letters rearranged gives 'CURATOR'.
'responsible for' acts as a link.
(Other definitions for curator that I've seen before include "Museum keeper" , "Museum officer" , "Museum's administrative head" , "Person in charge of a museum" , "Artistic director" .)