Big regal turn-out (5)
I believe the answer is:
large
'big' is the definition.
(I know that big can be written as large)
'regal turn-out' is the wordplay.
'turn-out' indicates an anagram.
'regal' is an anagram of 'LARGE'.
(Other definitions for large that I've seen before include "Bulky" , "Copious" , "Ample" , "Extensive" , "Immense" .)