Mother a bit wet, not all there (3)
I believe the answer is:
dam
'mother' is the definition.
(female parent of an animal)
'a bit wet not all there' is the wordplay.
'a bit wet' becomes 'damp' (damp can mean wet or moist).
'not all there' means to remove the last letter.
'damp' with its final letter taken away is 'DAM'.
(Other definitions for dam that I've seen before include "Female parent of an animal, especially livestock" , "Barrier to contain the flow of water" , "Embankment" , "Flood barrier" , "Wall restraining water" .)