When the girl returns, Don is out at the jeweller’s (7)
I believe the answer is:
diamond
'when the girl returns' is the definition.
I can't judge whether this definition defines the answer.
'don is out at the jeweller's' is the wordplay.
'is out' indicates an anagram (I've seen 'was out' mean this).
'at' is a charade indicator (letters next to each other) (I've seen this in other clues).
'the jeweller's' becomes 'diam' (I can't explain this - if you can you should give a lot more credence to this answer).
'don' anagrammed gives 'ond'.
'ond' after 'diam' is 'DIAMOND'.
Can you help me to learn more?
(Other definitions for diamond that I've seen before include "Sixtieth" , "The hard stuff" , "(Usually colourless) gemstone - card" , "60th anniversary" , "Small type" .)