Dam that is put back in Western Region (4)
I believe the answer is:
weir
'dam' is the definition.
(a weir is a type of dam)
'that is put back in western region' is the wordplay.
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'put back' says the letters should be written in reverse.
'in' is an insertion indicator.
'western region' becomes 'wr' (this could be a standard abbreviation I've not previously seen).
'ie' back-to-front is 'ei'.
'ei' inserted into 'wr' is 'WEIR'.
Can you help me to learn more?
(Other definitions for weir that I've seen before include "Small dam built across river to regulate flow" , "Structure for raising water level in river" , "River flow regulator" , "Small dam to regulate water flow" , "Low dam across a river" .)