Auden’s first stylised poem contains rhythms in the middle at the original beat (1,5)
I believe the answer is:
a tempo
'at the original beat' is the definition.
(musical term for the rhythm a piece began in)
'auden's first stylised poem contains rhythms in the middle' is the wordplay.
'auden's first' becomes 'a' (1st letter of 'auden').
'stylised' indicates an anagram (a restyling of the letters).
'contains' means one lot of letters goes inside another.
'in the middle' means to look at the middle letters.
The central letter of 'rhythms' is 't'.
'poem' with letters rearranged gives 'empo'.
'empo' going around 't' is 'tempo'.
'a'+'tempo'='A TEMPO'
(Other definitions for a tempo that I've seen before include "In time (music)" , "At the previous speed (music)" , "Staff direction" .)