It obstructs a lady no end! (3)
I believe the answer is:
dam
'it obstructs a' is the definition.
The definition suggests a singular noun which matches the answer.
'lady no end' is the wordplay.
'lady' becomes 'dame' (synonyms).
'no end' means to remove the last letter.
'dame' with its final letter taken away is 'DAM'.
Can you help me to learn more?
(Other definitions for dam that I've seen before include "A mad sort of mum" , "Mad as an animal mum" , "A mad animal mother" , "Reservoir barrier" , "Hold back; mother" .)