Little son, terribly dreary, yet can be very optimistic (6-4)
I believe the answer is:
starry-eyed
'very optimistic' is the definition.
(I've seen this before)
'little son terribly dreary yet' is the subsidiary indication.
'little' indicates taking the first letters.
'terribly' is an anagram indicator.
The initial letter of 'son' is 's'.
'dreary'+'yet'='drearyyet'
'drearyyet' anagrammed gives 'tarryeyed'.
's'+'tarryeyed'='starryeyed'
'can be' is the link.
(Other definitions for starry-eyed that I've seen before include "Full of unsophisticated optimism" , "Out of touch with reality" , "Naively idealistic" , "Romantically optimistic" , "Naively enthusiastic" .)