Unprepared, I am quick with excuses, at heart (9)

I believe the answer is:

impromptu

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'unprepared' is the definition.
(impromptu means improvised or unprepared)

'i am quick with excuses at heart' is the wordplay.
'i am' becomes 'I'm' (contraction).
'quick with' becomes 'prompt' (I can't explain this - if you can you should give a lot more credence to this answer).
'at heart' means to look at the middle letters.
The middle letter of 'excuses' is 'u'.
'im'+'prompt'+'u'='IMPROMPTU'

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(Other definitions for impromptu that I've seen before include "unrehearsed piece" , "Without much planning" , "Unplanned; composition" , "Extemporaneous speech" , "Unprepared" .)

I've seen this clue in The Telegraph.

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