Sid backed position as cashier (8)
I believe the answer is:
displace
'cashier' is the definition.
(thesaurus)
'sid backed position' is the wordplay.
'backed' says the letters should be written backwards.
'position' becomes 'place' (synonyms).
'sid' back-to-front is 'dis'.
'dis'+'place'='DISPLACE'
'as' acts as a link.
(Other definitions for displace that I've seen before include "Move out of position" , "Oust" , "Remove from usual habitat" , "Supplant" , "Succeed" .)