Stone for engagement day no girl's sent back (7)
I believe the answer is:
diamond
'stone for engagement' is the definition.
'diamond' can be an answer for 'stone' (I've seen this before). I'm not sure about the 'for engagement' bit.
'day no girl's sent back' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'girl' becomes 'maid' (I've seen this in other clues).
'sent back' shows that the letters should be reversed in order.
'd'+'no'+'maid'='dnomaid'
'dnomaid' written backwards gives 'DIAMOND'.
Can you help me to learn more?
(Other definitions for diamond that I've seen before include "Very hard gem" , "Birthstone for April" , "nickname of US State" , "60th anniversary" , "The hard stuff" .)