The most common and most likely answer to this crossword clue is the 4 plus 6 letter word FULL CIRCLE
I've seen this crossword clue in The Times
F
U
L
L
C
I
R
C
L
E
More clues leading to the result FULL CIRCLE
Buddy explains!

The straight part
We know that "No seats to be had upstairs?/Back to the start?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case no seats to be had upstairs?/back to the start? = full circle.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.