Long to cover mature show (7)
I believe the answer is:
pageant
'show' is the definition.
(thesaurus)
'long to cover mature' is the wordplay.
'long' becomes 'pant' (I've seen this before).
'to cover' means one lot of letters goes inside another.
'mature' becomes 'age' ('age' can be a synonym of 'mature').
'pant' going around 'age' is 'PAGEANT'.
(Other definitions for pageant that I've seen before include "Costumed procession" , "Dramatic performance" , "Neat gap for colourful spectacle" , "Spectacular ceremony" , "Tableaux or dramatic scenes depicting historical events" .)