Splendid inside picture gallery (5)
I believe the answer is:
regal
'splendid' is the definition.
(thesaurus)
'inside picture gallery' is the wordplay.
'inside' indicates a hidden word.
'REGAL' is hidden in the letters of 'picture gallery'.
(Other definitions for regal that I've seen before include "Magnificent, dignified" , "Portable organ" , "befitting a king" , "Imperial, majestic" , "Royal - 5 down (anag)" .)