Brief joy on loo — 2 (6)
I believe the answer is:
boggle
'2' is the definition.
The definition and answer are not the same part of speech.
'brief joy on loo' is the wordplay.
'brief' means to remove the last letter (a shorter or briefer version of the word).
'joy' becomes 'glee' (I've seen this in another clue).
'on' means one lot of letters go next to another.
'loo' becomes 'bog' (I can't justify this - if you can you should give a lot more credence to this answer).
'glee' with its final letter taken off is 'gle'.
'gle' after 'bog' is 'BOGGLE'.
Can you help me to learn more?
(Other definitions for boggle that I've seen before include "Overcome with amazement" , "Confuse and fluster" , "Baffle or fluster" , "Be surprised or baffled" , "Be startled or baffled (colloq.)" .)