First class single left out (1-3)
I believe the answer is:
a-one
'first class' is the definition.
(A1 can mean excellent or first class)
'single left out' is the wordplay.
I cannot really see how this works, but
'single' could be 'one' (a single thing - only one) and 'one' is located in the answer.
A single letter 'a' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for a-one that I've seen before include "Of the highest quality" , "First class!" .)