Shrewd, generous customer is a dumper (3-6)
I believe the answer is:
fly-tipper
'a dumper' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'shrewd generous customer' is the wordplay.
'shrewd' becomes 'fly' (fly can mean shrewd or astute).
'generous customer' becomes 'tipper' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fly'+'tipper'='FLY-TIPPER'
'is' is the link.
Can you help me to learn more?
(Other definitions for fly-tipper that I've seen before include "Person dumping rubbish illegally" , "Illegal rubbish dumper" .)