Not forgetting, when making out the invoice? (6,4,7)
I believe the answer is:
taking into account
'not forgetting' is the definition.
I can't tell whether this definition defines the answer.
'making out the invoice?' is the wordplay.
I cannot really see how this works, but
'making' could be 'takingin' ('take in' can be a synonym of 'make') and 'takingin' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'invoice?' could be 'account' (synonyms) and 'account' is present in the answer.
A single letter 'o' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'when' is the link.
Can you help me to learn more?