Show many a true idea starts at birth (7)
I believe the answer is:
matinee
'show' is the definition.
(I know that matinee is a type of histrionics)
'many a true idea starts at birth' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'starts' says to take the initial letters.
'at birth' becomes 'nee' (I've seen this in another clue).
The first letters of 'true idea' is 'ti'.
'm'+'a'+'ti'+'nee'='MATINEE'
(Other definitions for matinee that I've seen before include "theatre performance" , "Afternoon performance (of play etc)" , "First jackets" , "After lunch, show" , "early "performance"" .)