Condition nursed by fantastic beast (6)
I believe the answer is:
agouti
'beast' is the definition.
(agouti is a kind of beast)
'condition nursed by fantastic' is the wordplay.
I cannot really understand how this works, but
'condition' could be 'gout' (gout is a kind of condition) and 'gout' is found in the answer.
The remaining letters 'ai' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for agouti that I've seen before include "S.Amer. rodent" , "South American rodent" , "Creature" , "Rabbit-sized rodent of South America valued as food" , "S. American burrowing rodent" .)