Spontaneous date started with pre-nup Tom prepared (10)

I believe the answer is:

unprompted

'spontaneous' is the definition.
(similar in meaning)

'date started with pre-nup tom prepared' is the wordplay.
'started with' indicates taking the first letters (I've seen 'start with' mean this).
'prepared' indicates anagramming the letters.
The first letter of 'date' is 'd'.
'd'+'pre'+'nup'+'tom'='dprenuptom'
'dprenuptom' is an anagram of 'UNPROMPTED'.

I've seen this clue in The Independent.

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