Study of pressure, except on old Metropolitan Line (9)

I believe the answer is:

barometry

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'study of pressure' is the definition.
The definition suggests a singular noun which matches the answer.

'except on old metropolitan line' is the wordplay.
'except' becomes 'bar' ('bar' can mean 'except' or 'not including').
'on' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'metropolitan' becomes 'met' (I can't justify this - if you can you should give a lot more credence to this answer).
'line' becomes 'ry' (abbreviation for railway).
'bar'+'o'+'met'+'ry'='BAROMETRY'

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I've seen this clue in The Independent.

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