Fierce cause of much agony for a doctor (6)
I believe the answer is:
dragon
'fierce cause' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'much agony for a doctor' is the wordplay.
'much' means to remove the last letter (much but not all of the word).
'for' says to put letters next to each other (I've seen this in other clues).
'a doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'agony' with its last letter removed is 'agon'.
'agon' put after 'dr' is 'DRAGON'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for dragon that I've seen before include "Mythical monster - large lizard" , "Fire-breathing creature slain by St George" , "Fire-breathing reptile, usually with wings" , "Fiery reptile or person" , "Intractable person" .)