Ideal location: best one's in centre of square (6)
I believe the answer is:
utopia
'ideal location' is the definition.
(imaginary perfect place)
'best one's in centre of square' is the wordplay.
'best' becomes 'top' (top can mean the very best).
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'centre of' indicates the central letters.
The middle of 'square' is 'ua'.
'top'+'i'='topi'
'topi' put within 'ua' is 'UTOPIA'.
(Other definitions for utopia that I've seen before include "Sir Thomas More's ideal society" , "Ideal country" , "Thomas More's ideal state" , "Place of ideal perfection" , "of limited interest to G&S" .)