Sensational media boss became sickly looking (8)
I believe the answer is:
yellowed
'sickly looking' is the definition.
(jaundiced)
'sensational media boss' is the wordplay.
'sensational' becomes 'yellow' (similar in meaning).
'media boss' becomes 'ed' (short for editor).
'yellow'+'ed'='YELLOWED'
'became' is the link.
(Other definitions for yellowed that I've seen before include "Discoloured with age" , "showed effects of age" , "Like discoloured paper" , "Became jaundiced in appearance?" .)