In operation over the brook (2,6)

I believe the answer is:

on stream

'in operation' is the definition.
(I've seen this before)

'over the brook' is the wordplay.
'over' becomes 'on' (I've seen this in other clues**).
'the brook' becomes 'stream' (I've seen this before).
'on'+'stream'='ON-STREAM'

(Other definitions for on stream that I've seen before include "In operation or existence" , "in production" , "A monster (anag.)" , "In operation; storeman (anag.)" .)

I've seen this clue in The Telegraph.

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