Tops of mosques in Riyadh require gold reflector (6)
I believe the answer is:
mirror
'reflector' is the definition.
(I know that mirror is a type of reflector)
'tops of mosques in riyadh require gold' is the wordplay.
'tops of' indicates taking the first letters.
'gold' becomes 'or' ('or' is the heraldic name for gold).
The initial letters of 'mosques in riyadh require' is 'mirr'.
'mirr'+'or'='MIRROR'
(Other definitions for mirror that I've seen before include "Way of seeing oneself" , "Reflecting surface" , "Daily paper" , "Image reflector" , "It reflects images" .)