Heard boy is a star (3)
I believe the answer is:
sun
'is a star' is the definition.
(I've seen this before)
'heard boy' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'boy' becomes 'son' (synonyms).
'son' is a homophone of 'SUN'.
(Other definitions for sun that I've seen before include "Helios, in Greek myth" , "Bask" , "Rag" , "Star around which the earth travels" , "Nice weather" .)