Medium-busy, emitting dangerous particles (11)
I believe the answer is:
radioactive
'emitting dangerous particles' is the definition.
(I've seen this before)
'medium-busy' is the wordplay.
'medium' becomes 'radio' (I've seen this in another clue).
'busy' becomes 'active' (similar in meaning).
'radio'+'active'='RADIOACTIVE'
(Other definitions for radioactive that I've seen before include "Emitting ionising particles" , "hot" , "description of nuclear waste" , "putting out unhealthy material" , "Emitting dangerous particles" .)