Son worries about one nightmare essentially that’s most terrifying (8)
I believe the answer is:
scariest
'most terrifying' is the definition.
(I've seen this before)
'son worries about one nightmare essentially' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'worries' becomes 'cares' (I've seen this before).
'about' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'essentially' means to look at the middle letters.
The middle letter of 'nightmare' is 't'.
'cares' going around 'i' is 'caries'.
's'+'caries'+'t'='SCARIEST'
'that's' acts as a link.
(Other definitions for scariest that I've seen before include "Most terrifying" , "most alarming" , "Most frightening" .)