Starts with engineer implying river should have a dam (4)
I believe the answer is:
weir
'dam' is the definition.
(a weir is a type of dam)
'starts with engineer implying river' is the wordplay.
'starts' says to take the initial letters.
The initial letters of 'with engineer implying river' is 'WEIR'.
'should have a' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
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" , "feature of some rivers" , "Small dam to regulate water flow" , "Barrier" , "fish may be trapped in this enclosure" .)