Son going to Underworld in dark glasses (6)
I believe the answer is:
shades
'dark glasses' is the definition.
('shades' can be a synonym of 'dark glasses')
'son going to underworld' is the wordplay.
'son' becomes 's'.
'going to' says to put letters next to each other.
'underworld' becomes 'hades' ('Hades' can be a synonym of 'underworld').
's'+'hades'='SHADES'
'in' is the link.
(Other definitions for shades that I've seen before include "blinds?" , "Sunglasses, colloquially" , "Old ghosts or modern sunglasses" , "glasses?" , "Sunglasses (informal)" .)