Fade away in barren land, becoming more sullen (7)
I believe the answer is:
moodier
'more sullen' is the definition.
('moody' can be a synonym of 'sullen')
'fade away in barren land' is the wordplay.
'fade away' becomes 'die' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'barren land' becomes 'moor' (moor is a kind of land. I am not sure about the 'barren' bit.).
'die' inserted within 'moor' is 'MOODIER'.
'becoming' acts as a link.
Can you help me to learn more?
(Other definitions for moodier that I've seen before include "more gloomy" , "More sullen" .)