The most common and most likely answer to this crossword clue is the 2 plus 3 plus 2 letter word AD LIB AD
I've seen this crossword clue in The Times
A
D
L
I
B
A
D
Buddy explains!
The straight part
We know that "spontaneously" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case spontaneously = ad lib ad.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.