Would one look vacant when asked to spell them? (5,5)

'asked to spell them?' is the definition.
I can't tell whether this definition defines the answer.

'would one look vacant' is the wordplay.
I cannot really see how this works, but
'would' could be 'd' (I've seen this in other clues) and 'd' is found in the answer.
'one' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
'vacant' could be 'empty' (similar in meaning) and 'empty' is found in the answer.
This explanation may well be incorrect...

'when' acts as a link.

Can you help me to learn more?