Drama of T.Rex getting on Glastonbury at last (7)
I believe the answer is:
tragedy
'drama' is the definition.
(tragedy is a type of drama)
't rex getting on glastonbury at last' is the wordplay.
'rex' becomes 'R' (abbreviation e.g. GR=George Rex, King George).
'getting on' becomes 'aged' (getting on can mean getting older).
'at last' indicates one should take the final letters.
The final letter of 'glastonbury' is 'y'.
't'+'r'+'aged'+'y'='TRAGEDY'
'of' acts as a link.
(Other definitions for tragedy that I've seen before include "Hamlet, say" , "Such a story ends unhappily" , "literary genre Aristotle analysed" , "King Lear, perhaps" , "Catastrophic event that causes great sadness" .)