Wild roo, seen in Mull, causing mirth (8)
I believe the answer is:
humorous
'mull causing mirth' is the definition.
I can't tell whether this defines the answer.
'wild roo seen in' is the wordplay.
'wild' is an anagram indicator.
'seen' becomes 'humus' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'roo' anagrammed gives 'oro'.
'oro' inserted within 'humus' is 'HUMOROUS'.
Can you help me to learn more?
(Other definitions for humorous that I've seen before include "Comic" , "Causing amusement" , "Risible" , "Comical or amusing" , "Funny, amusing" .)