Write first letter describing their impatient personality? (5)

I believe the answer is:

typea

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'their impatient personality?' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'write first letter' is the wordplay.
'write' becomes 'type' (typing is a kind of writing).
'first letter' becomes 'a' (in the alphabet).
'type'+'a'='TYPEA'

'describing' is the link.

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I've seen this clue in The Irish Times.

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