Son was worried, frightened (6)
I believe the answer is:
scared
'frightened' is the definition.
('scare' can be a synonym of 'frighten')
'son was worried' is the wordplay.
'son' becomes 's'.
'was worried' becomes 'cared' (to care is to worry).
's'+'cared'='SCARED'
(Other definitions for scared that I've seen before include "Nervous" , "Frightened or apprehensive" , "Frightened, alarmed" , "in fright" , "Thing which precipitates an event" .)