Dead characters seen in Forever Young (4)
I believe the answer is:
very
'dead' is the definition.
('dead' can mean extremely, eg 'dead good')
'characters seen in forever young' is the wordplay.
'characters seen in' indicates the answer is hidden within the clue.
'VERY' is hidden in the letters of 'forever young'.
(Other definitions for very that I've seen before include "Actual; in a high degree" , "Extremely - light" , "Exceedingly" , "To a high degree or extent" , "light, perhaps" .)