One is extremely tense holding the butterflies presumably (1,3,2,6)
I believe the answer is:
a bag of nerves
'one is extremely tense holding' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'butterflies presumably' is the wordplay.
I cannot really understand how this works, but
'butterflies' could be 'nerves' (I've seen this in another clue) and 'nerves' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'the' acts as a link.
Can you help me to learn more?
(Another definition for a bag of nerves that I've seen is " Person who is extremely timid or tense".)