One who composed message on Mother’s Day gift? Revolting, oily stuff! (8)
I believe the answer is:
bergamot
'oily stuff' is the definition.
(bergamot is an oily substance)
'one who composed message on mother's day gift?' is the wordplay.
'one who composed' becomes 'Berg' (Alban Berg was a composer).
'message' becomes 'AM' (AM is an example).
'on' says to put letters next to each other.
'mother's day gift?' becomes 'ot' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'berg'+'am'+'ot'='BERGAMOT'
'revolting' is the link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for bergamot that I've seen before include "perfume" , "Variety of pear" , "Kind of orange - great mob (anag)" , "essential oil" , "Oily substance extracted oranges, used in Earl Grey tea" .)