Being so spoils the fun a little bit (5)
I believe the answer is:
unfit
'being so' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'spoils the fun a little bit' is the wordplay.
'spoils' indicates an anagram.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'little' means to remove the last letter (the word is not quite complete).
'bit' becomes 'i' (resembles 1, a binary digit or bit).
't'+'fun'+'a'='tfuna'
'tfuna' with its final letter removed is 'tfun'.
'tfun'+'i'='tfuni'
'tfuni' with letters rearranged gives 'UNFIT'.
Can you help me to learn more?
(Other definitions for unfit that I've seen before include "Not in good physical shape (5)" , "Not in good physical condition, or not suitable" , "Inappropriate" , "In poor condition" , "Ill - out of shape" .)