Weaken a magnet when picked up (5)
I believe the answer is:
abate
'weaken' is the definition.
(to abate is to weaken or become less intense)
'a magnet when picked up' is the wordplay.
'magnet' becomes 'bait' (both can mean something that draws or lures).
'when picked up' shows a homophone (sound like) (pick up can mean to hear).
'bait' sounds like 'bate'.
'a'+'bate'='ABATE'
(Other definitions for abate that I've seen before include "Demolish" , "Let up or die away" , "Lessen - cease" , "Go down like storm" , "Put an end to" .)