George Martin, for one, mixed up record (8)
I believe the answer is:
producer
'george martin' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'one mixed up record' is the wordplay.
'one mixed' is an anagram indicator.
'up'+'record'='uprecord'
'uprecord' with letters rearranged gives 'PRODUCER'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for producer that I've seen before include "Manufacturer; film overseer" , "One finds the finance for films" , "Person responsible for putting on a play" , "Creator; supervisor (of programme, play)" , "Procured for film or play organiser" .)