Instrument in transit, a radiometer (5)
I believe the answer is:
sitar
'instrument' is the definition.
(sitar is a kind of instrument)
'in transit a radiometer' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SITAR' is hidden in the letters of 'transit a radiometer'.
(Other definitions for sitar that I've seen before include "Guitar-like instrument" , "Hindu stringed instrument" , "Long necked and often plucked" , "music-maker" , "Indian musical instrument like long-necked lute" .)