Birds - they pick up heavy weights (6)
I believe the answer is:
cranes
'birds' is the definition.
(crane is a kind of bird)
I can't explain the remainder of the clue.
(Other definitions for cranes that I've seen before include "demoiselles" , "Lifting machines and long-necked birds" , "Makes an effort to see" , "Lifting machines and wading birds" , "Long-necked wading birds" .)