Useful when writing streams of letters? (7,4)

'streams of letters?' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'useful when writing' is the wordplay.
I cannot quite see how this works, but
'writing' could be 'hand' ('hand' can refer to someone's handwriting) and 'hand' is located in the answer.
The remaining letters 'flowing' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?