Performing stage work in simple key, including tango (9)
I believe the answer is:
operating
'performing' is the definition.
(I've seen this before)
'stage work in simple key including tango' is the wordplay.
'stage work' becomes 'opera' (I've seen this before).
'simple key' becomes 'g' (musical key. I am not sure about the 'simple' bit.).
'including' is an insertion indicator.
'tango' becomes 'T' (phonetic alphabet: alpha, bravo, charlie etc.).
'opera'+'in'+'g'='operaing'
'operaing' going around 't' is 'OPERATING'.
Can you help me to learn more?
(Other definitions for operating that I've seen before include "Active" , "Directing or controlling, machinery say" , "Performing surgery" , "(Of machine) running" , "at work" .)