Gloomy and sad, I'm about to reach the half century (6)
I believe the answer is:
dismal
'gloomy' is the definition.
(synonyms)
'sad i'm about to reach the half century' is the wordplay.
'about' indicates anagramming the letters.
'to reach' says to put letters next to each other.
'the half century' becomes 'L' (Roman numeral for 50).
'sad'+'im'='sadim'
'sadim' anagrammed gives 'disma'.
'disma'+'l'='DISMAL'
'and' is the link.
(Other definitions for dismal that I've seen before include "Showing gloom, dreary" , "Gloomy and depressing like winter weather" , "Miserable" , "Dreary, cheerless" , "'Gloomy, grim (6)'" .)