Out climbing, finds radioactive element? (5)
I believe the answer is:
niton
'radioactive element?' is the definition.
(I've seen this before)
'out climbing' is the wordplay.
'out' becomes 'not in' (I've seen this before).
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'notin' written backwards gives 'NITON'.
'finds' is the link.
(Other definitions for niton that I've seen before include "Radioactive element" , "used for radiotherapy" .)