Ray might be doing this in mum's wig, dancing with no uniform (8)
I believe the answer is:
swimming
'ray might be doing this' is the definition.
(rays swim)
'dancing with no uniform' is the wordplay.
'dancing' becomes 'swing' (swing is a kind of dancing).
'with' indicates putting letters inside.
'no uniform' becomes 'imm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'swing' placed around 'imm' is 'SWIMMING'.
'in mum's wig' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for swimming that I've seen before include "Bathing" , "Moving through water" , "Doing crawl" , "Filled with tears" , "Travelling through water" .)