Fit au pair periodically used car (6)
I believe the answer is:
jaguar
'car' is the definition.
(I have seen 'posh car' mean 'jaguar' so perhaps 'car' could also mean 'jaguar')
'fit au pair periodically' is the wordplay.
'fit' becomes 'jag' (I can't justify this - if you can you should believe this answer much more).
'periodically' indicates alternate letters (letters must be selected 'every so often').
'au'+'pair'='aupair'
The alternating letters of 'aupair' are 'uar'.
'jag'+'uar'='JAGUAR'
'used' is the link.
Can you help me to learn more?
(Other definitions for jaguar that I've seen before include "Wild cat from S America, or fast car" , "Big cat of tropical America" , "Largest American big cat" , "S.Amer. wild cat" , "Large feline mammal" .)