Cheap device for identifying stream by church (9)
I believe the answer is:
pinchbeck
'cheap' is the definition.
(I've seen this before)
'device for identifying stream by church' is the wordplay.
'device for identifying' becomes 'PIN' (personal identification number).
'stream' becomes 'beck' (northern English dialect term for 'stream').
'by' says to put letters next to each other.
'church' becomes 'ch' (abbreviation on maps etc).
'beck' put after 'ch' is 'chbeck'.
'pin'+'chbeck'='PINCHBECK'
(Other definitions for pinchbeck that I've seen before include "Imitation gold" , "Cheap (alloy); sham" .)