Stop composer penning endings to round off sonata (4,4)

I believe the answer is:

hold fast

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'endings to round off sonata' is the definition.
I can't tell whether this defines the answer.

'stop composer penning' is the wordplay.
'stop' becomes 'dfa' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'composer' becomes 'holst' (Gustav Holst).
'penning' means one lot of letters goes inside another.
'dfa' put inside 'holst' is 'HOLD-FAST'.

Can you help me to learn more?

I've seen this clue in The Guardian.

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