Au pair starts one quickly (5)
I believe the answer is:
apace
'quickly' is the definition.
(synonyms)
'au pair starts one' is the wordplay.
'starts' indicates taking the first letters.
'one' becomes 'ace' (ace has a value of one in some card games).
The first letters of 'au pair' is 'ap'.
'ap'+'ace'='APACE'
(Other definitions for apace that I've seen before include "flat out" , "(Move on) swiftly" , "At a rapid rate" , "What's quickly" , "Swiftly, quickly" .)