He knows how to wipe a joint (7)
I believe the answer is:
plumber
'he knows how to wipe a joint' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he' is the wordplay.
I cannot quite see how this works, but
'he' could be 'ber' and 'ber' is found in the answer.
The remaining letters 'plum' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for plumber that I've seen before include "tradesperson" , "One fits water and gas pipes" , "He fixes water-pipes etc in building" , "Repairer of pipes" , "Pipe-fitter" .)