Smallest of visible asteroids, shrinking (5)
I believe the answer is:
least
'smallest' is the definition.
(synonyms)
'of visible asteroids shrinking' is the wordplay.
'of' indicates the answer is hidden within the clue.
'LEAST' can be found hidden inside 'visible asteroids shrinking'.
(Other definitions for least that I've seen before include "Smallest, slightest" , "No more petty" , "Superlatively little" , "superlatively small" , "Most unimportant" .)