Perfect location, you heard, the best: central Siam (6)
I believe the answer is:
utopia
'perfect location' is the definition.
(I've seen this in another clue)
'you heard the best central siam' is the wordplay.
'you heard' becomes 'U' (homophone of 'you').
'the best' becomes 'top' (top can mean the very best).
'central' means to look at the middle letters.
The middle letters of 'siam' are 'ia'.
'u'+'top'+'ia'='UTOPIA'
(Other definitions for utopia that I've seen before include "Impracticable state of perfection" , "Sir Thomas More's perfect place" , "Imaginary place where everything is perfect" , "impossible to find" , "Literally nowhere" .)