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