Opera reflected irrational score markings (5)
I believe the answer is:
tempi
'score markings' is the definition.
(I've seen this in another clue)
'opera reflected irrational' is the wordplay.
'opera' becomes 'met' (Metropolitan Opera in New York).
'reflected' is a reversal indicator (letters reflected back).
'irrational' becomes 'pi' (an irrational number).
'met' backwards is 'tem'.
'tem'+'pi'='TEMPI'
(Other definitions for tempi that I've seen before include "Speeds, rhythms of musical pieces" , "Times" , "Speeds (music)" , "Beats" , "Speeds for musicians" .)