Countless flyers imported, all about milk supplier (5,4)
I believe the answer is:
dairy farm
'milk supplier' is the definition.
(a dairy farm supplies milk)
'countless flyers imported all about' is the wordplay.
'countless' becomes 'myriad' (synonyms).
'flyers' becomes 'RAF' (Royal Air Force).
'imported' is an insertion indicator.
'all about' shows that the letters should be reversed in order.
'myriad' enclosing 'raf' is 'mrafyriad'.
'mrafyriad' written backwards gives 'DAIRY FARM'.
(Other definitions for dairy farm that I've seen before include "Cattle are kept for milk here" , "cheese-shop supplier" , "rural building" , "Milk-producing concern" , "Supplier of milk products" .)