Impatient principal might give out but it's perfect (5)
I believe the answer is:
ideal
'it's perfect' is the definition.
(similar in meaning)
'impatient principal might give out' is the wordplay.
'principal' suggests taking the first letters (the principal letter is the first letter).
'might give out' becomes 'deal' (I am not sure about this - if you are sure you should believe this answer much more).
The first letter of 'impatient' is 'i'.
'i'+'deal'='IDEAL'
'but' is the link.
Can you help me to learn more?
(Other definitions for ideal that I've seen before include "Model of excellance" , "Benchmark" , "supreme" , "without flaws" , "Perfectly suited" .)