Wise guy loses the head - it's the last straw for the bar restricting entry (3,5)
I believe the answer is:
age limit
'the last straw for the bar restricting entry' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'wise guy loses the head it's' is the wordplay.
I cannot quite see how this works, but
'it's' could be 'it' (I've seen this in other clues) and 'it' is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for age limit that I've seen before include "Maturity restriction" , "Regulation based on year of birth that sets down when one can no longer do something" , "terminus a quo, say" , "I am legit (anag)" , "Bar to those too old or too young" .)