Reverse is large and extremely shiny (7)
I believe the answer is:
silvery
'shiny' is the definition.
(I know that silvery is a more specific form of the attribute shiny)
'reverse is large and extremely' is the wordplay.
'reverse' shows that the letters should be reversed in order.
'large' becomes 'L' (eg in clothes sizes).
'and' means one lot of letters go next to another.
'extremely' becomes 'very' (very can mean extremely or considerably).
'is' written backwards gives 'si'.
'si'+'l'+'very'='SILVERY'
(Other definitions for silvery that I've seen before include "Like one precious metal" , "Having a clear ringing tone; like a precious metal" , "Epithet for the moon" , "Elementary description of moon" , "Whitish; gently ringing" .)