Starts to tire of pa's irritatingly childish question (5)
I believe the answer is:
topic
'question' is the definition.
(I've seen this before)
'starts to tire of pa's irritatingly childish' is the wordplay.
'starts to' suggests taking the first letters.
The initial letters of 'tire of pas irritatingly childish' is 'TOPIC'.
(Other definitions for topic that I've seen before include "Subject (eg, of text)" , "Talking point" , "'Theme, subject (5)'" , "Subject (of a talk, etc)" , "Content of argument" .)