Fewer than one in Vichy turned Communist (5)
I believe the answer is:
under
'fewer than' is the definition.
'one in vichy turned communist' is the wordplay.
'one in vichy' becomes 'un' (I can't justify this - if you can you should give a lot more credence to this answer).
'turned' is a reversal indicator.
'communist' becomes 'red' (red can informally mean communist).
'red' written backwards gives 'der'.
'un'+'der'='UNDER'
Can you help me to learn more?
(Other definitions for under that I've seen before include "See 16 Down" , "See 7" , "Being short" , "Managed by" , "Covered" .)