Dam is not entirely odd (4)
I believe the answer is:
weir
'dam' is the definition.
(a weir is a type of dam)
'not entirely odd' is the wordplay.
'not entirely' means to remove the last letter (not all of the word).
'odd' becomes 'weird' (I've seen this before).
'weird' with its last letter taken off is 'WEIR'.
'is' is the link.
(Other definitions for weir that I've seen before include "Feature of river" , "Madagascan primate" , "Dam on a stream to control water level or flow" , "type of block" , "Low dam across a river" .)