Maintains a high level opening pump behind new spout (4,2)

I believe the answer is:

tops up

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'maintains a high level' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'opening pump behind new spout' is the wordplay.
'opening' suggests taking the first letters.
'behind' means one lot of letters go next to another.
'new' indicates an anagram.
The initial letter of 'pump' is 'p'.
'spout' anagrammed gives 'topsu'.
'p' put after 'topsu' is 'TOPS UP'.

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I've seen this clue in the Irish Times (Crosaire) and the Irish Times.

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