Misses boomerangs collection that's found next to mine (4,4)
I believe the answer is:
slag heap
'that's found next to mine' is the definition.
(refuse heap next to a mining site)
'misses boomerangs collection' is the wordplay.
'misses' becomes 'gals' ('gal' can be a synonym of 'miss').
'boomerangs' shows that the letters should be reversed in order (letters rebound back like a boomerang).
'collection' becomes 'heap' (heap is a kind of collection).
'gals' written backwards gives 'slag'.
'slag'+'heap'='SLAG-HEAP'
(Other definitions for slag heap that I've seen before include "Hill of mining 7" , "Mound of coalmining waste" , "Hill of mining refuse" , "Pile of mine refuse" , "Pile of waste matter from coal mining" .)