Opening measuring device, finally place extra fitting (once) (6)

'extra fitting once' is the definition.
(meet could historically mean fitting or apt)

'opening measuring device finally place' is the wordplay.
'opening' means one lot of letters goes inside another (some letters open a gap in others).
'measuring device' becomes 'meter' (device for measuring something, eg a thermometer).
'finally' indicates one should take the final letters.
The final letter of 'place' is 'e'.
'meter' enclosing 'e' is 'MEETER'.