Barman has endless supply of drink knocked back, for example, by mooch (8)
I believe the answer is:
scrounge
'mooch' is the definition.
(thesaurus)
'barman has endless supply of drink knocked back for example' is the wordplay.
I cannot really see how this works, but
'barman' could be 'sc' (I've seen this in other clues) and 'sc' is located in the answer.
'of' could be 'o' and 'o' is present in the answer.
This explanation may well be incorrect...
'by' acts as a link.
Can you help me to learn more?
(Other definitions for scrounge that I've seen before include "Seek something free from others" , "bum" , "Acquire by begging" , "Cur's gone (anag) - beg" , "Obtain (something) for free" .)