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' (short for Latin 'id est', 'that is').
'put back' says the letters should be written backwards.
'in' indicates putting letters inside.
'western region' becomes 'wr' (this could be a standard abbreviation of which I'm unaware).
'ie' in reverse letter order is 'ei'.
'ei' going inside '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" .)