Weirdo's old shed finally decorated (3,3,3)
I believe the answer is:
odd one out
'weirdo's' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'old shed finally decorated' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'finally' suggests the final letters.
'decorated' becomes 'doneout' (I can't justify this - if you can you should give a lot more credence to this answer).
The final letter of 'shed' is 'd'.
'o'+'d'+'doneout'='ODD ONE OUT'
Can you help me to learn more?
(Other definitions for odd one out that I've seen before include "Individualist" , "exception" , "Thing or person contrasting with others" , "Different individual" .)