Sullen and low, ready to finish (5)
I believe the answer is:
moody
'sullen' is the definition.
(I know that sullen can be written as moody)
'low ready to finish' is the wordplay.
'low' becomes 'moo' (both can mean to make a cow noise).
'ready' becomes 'dy' (I can't explain this - if you can you should give a lot more credence to this answer).
'to finish' means one lot of letters go next to another (some letters go after or 'finish' others).
'moo'+'dy'='MOODY'
'and' acts as a link.
Can you help me to learn more?
(Other definitions for moody that I've seen before include "Changeable, sullen" , "Irritable" , "Down in the dumps" , "Mercurial" , "Temperamental -- capricious" .)