Ancient king, very large and fat, ditching horse (5)
I believe the answer is:
osric
'ancient king' is the definition.
The definition suggests a singular noun which matches the answer.
'very large and fat ditching horse' is the wordplay.
'very large' becomes 'os' (abbreviation for oversize).
'and' means one lot of letters go next to another.
'fat' becomes 'rich' (as in rich foods).
'ditching' indicates named letters should be taken away.
'horse' becomes 'H' (synonyms).
'rich' with 'h' taken away is 'ric'.
'os'+'ric'='OSRIC'
Can you help me to learn more?
(Other definitions for osric that I've seen before include "Danish courtier" , "Shakespearean courtier" , "Shakespearian" .)