Heard boy is star (3)
I believe the answer is:
sun
'is star' is the definition.
(the Sun is a star)
'heard boy' is the wordplay.
'heard' shows a homophone (sound like).
'boy' becomes 'son' (male child).
'son' sounds like 'SUN'.
(Other definitions for sun that I've seen before include "Nearest star to Earth" , "Nice weather" , "Star around which the earth travels" , "Dawn riser" , "Source of daylight" .)