Informal market offering mostly crap, perhaps: footwear, booze (3,4,4)
I believe the answer is:
car boot sale
'informal market' is the definition.
(I know that car boot sale is a type of informal market)
'mostly crap perhaps footwear booze' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'perhaps' indicates an anagram.
'footwear' becomes 'boots' (I've seen this before).
'booze' becomes 'ale' (type of alcohol).
'crap' with its final letter taken away is 'cra'.
'cra' anagrammed gives 'car'.
'car'+'boots'+'ale'='CAR-BOOT SALE'
'offering' is the link.
(Other definitions for car boot sale that I've seen before include "Fair" , "Second hand market" , "Event involving trade conducted from the back of one's vehicle" , "Vending of second-hand goods from private vehicles" , "Cheap market" .)