Use a spade on the thing in doing it (3,4)

'use a spade on the thing in doing it' is the definition.
I can't judge whether this definition defines the answer.

'use a spade on the thing in' is the wordplay.
I cannot quite see how this works, but
'on' could be 'to' and 'to' is present in the answer.
'in' is found in the answer.
'use a spade' could be 'dig' (I've seen this before) and 'dig' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...

Can you help me to learn more?