My return during her performance - that's poetry (5)
I believe the answer is:
rhyme
'that's poetry' is the definition.
(rhyme is a type of poetry)
'my return during her performance' is the wordplay.
'return' says the letters should be written in reverse.
'during' means one lot of letters goes inside another.
'performance' is an anagram indicator.
'my' backwards is 'ym'.
'her' anagrammed gives 'rhe'.
'ym' inserted inside 'rhe' is 'RHYME'.
(Other definitions for rhyme that I've seen before include "anticipated by a nursery perhaps" , "Similarity of word-sounds" , "Short piece of verse" , "Correspondence in the sounds of two or more words" , "Sound correspondence" .)