Moocher may steal up on boatman (8)
I believe the answer is:
borrower
'boatman' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'moocher may steal up on' is the wordplay.
'moocher may' becomes 'rower' (I can't justify this - if you can you should give a lot more credence to this answer).
'steal' becomes 'rob' (robbing is a kind of stealing).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'on' means one lot of letters go next to another.
'rob' written backwards gives 'bor'.
'rower' put after 'bor' is 'BORROWER'.
Can you help me to learn more?
(Other definitions for borrower that I've seen before include "One intends to return things" , "who has ignored advice of Polonius" , "One having interest" , "One taking out a loan" , "one temporarily in debt" .)