What it is to call anybody a bit of a loon (8)
I believe the answer is:
personal
'what it is' is the definition.
I can't judge whether this definition defines the answer.
'call anybody a bit of a loon' is the wordplay.
'call anybody' becomes 'persona' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a bit of' indicates taking the first letters.
The first letter of 'loon' is 'l'.
'persona'+'l'='PERSONAL'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for personal that I've seen before include "Private - bodily" , "Relating to the individual" , "Of private concern" , "Such a rude remark" , "Offensively intrusive" .)