Queen once performed last in Glasto (4)
I believe the answer is:
dido
'queen' is the definition.
(legendary queen of Carthage)
'performed last in glasto' is the wordplay.
'performed' becomes 'did' ('do' can be a synonym of 'perform').
'last in' indicates one should take the final letters.
The last letter of 'glasto' is 'o'.
'did'+'o'='DIDO'
'once' acts as a link.
(Other definitions for dido that I've seen before include "Ancient queen of Carthage" , "Carthaginian queen linked to Aeneas" , "Lover of Aeneas" , "Queen < singer" , "She loved Aeneas" .)