The number of people in operation theatre (12)
I believe the answer is:
anaesthetist
'number of people in operation theatre' is the definition.
'anaesthetist' can be an answer for 'number' (I've seen this before). I'm not certain of the 'of people in operation theatre' bit.
'the' is the wordplay.
I cannot quite understand how this works, but
an anagram of 'the' is 'het' which is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for anaesthetist that I've seen before include "Hospital theatre worker" , "Number" , "One preparing patients for operations" .)