Accumulate boomerangs in circle (4)
I believe the answer is:
loop
'circle' is the definition.
(looping is a kind of circling)
'accumulate boomerangs' is the wordplay.
'accumulate' becomes 'pool' (I've seen this before).
'boomerangs' says the letters should be written backwards (letters rebound back like a boomerang).
'pool' reversed gives 'LOOP'.
'in' acts as a link.
(Other definitions for loop that I've seen before include "Oval shape" , "Contraception device - hearing aid" , "the knowledgeable are in it" , "Computer subroutine" , "Figure made by curved line crossing itself" .)