Just toting unusual heft spontaneously (2,3,3)
I believe the answer is:
on the fly
'spontaneously' is the definition.
Both the definition and answer are adverbs. Maybe there's an association between them I don't understand?
'just toting unusual heft' is the wordplay.
I cannot really see how this works, but
an anagram of 'heft' is 'thef' which is present in the answer.
'just' could be 'only' (synonyms) and 'only' is found in the leftover letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for on the fly that I've seen is " In a hurry - lofty hen (anag)".)