Working, approach Emergency Room about temperature measurement device (9)

'measurement device' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'working approach emergency room about temperature' is the wordplay.
'working' becomes 'on' (e.g. a machine is working or on).
'approach' becomes 'come' (e.g. as imperative).
'emergency room' becomes 'ER' (abbreviation popularized by US TV series).
'about' is a charade indicator (letters next to each other).
'temperature' becomes 't' (abbreviation).
'er' put after 't' is 'ter'.
'on'+'come'+'ter'='ONCOMETER'

Can you help me to learn more?