Fifty is the difference between your choices if threatened (5-2-6)
I believe the answer is:
fight-or-flight
'difference between your choices if threatened' is the definition.
I know nothing about this answer so I can't judge whether this works.
'fifty is the' is the wordplay.
I cannot really understand how this works, but
'fifty' could be 'li' (I've seen this in other clues) and 'li' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for fight-or-flight that I've seen is " Instinctive response to a sudden threat".)