Switch got reversed with joy mostly (6)
I believe the answer is:
toggle
'switch' is the definition.
(I know that toggle is a type of switch)
'reversed with joy mostly' is the wordplay.
'reversed with' becomes 'tog' (I can't explain this - if you can you should believe this answer much more).
'joy' becomes 'glee' (I've seen this before).
'mostly' means to remove the last letter (most but not all of the word).
'glee' with its last letter taken away is 'gle'.
'tog'+'gle'='TOGGLE'
'got' acts as a link.
Can you help me to learn more?
(Other definitions for toggle that I've seen before include "Peg-like button" , "Duffel coat fastener" , "Short bar acting as button" , "Coat fastener; computer switch" , "Fastener; type of switch" .)