Those people around box office wanting to take in pornographic picture (3,4,3)
I believe the answer is:
the blue boy
'picture' is the definition.
(Thomas Gainsborough painting)
'those people around box office wanting to take in pornographic' is the wordplay.
'those people' becomes 'they' (I've seen this before).
'around' is an insertion indicator.
'box office' becomes 'bo' (entertainment industry acronym).
'wanting to take in' is an insertion indicator.
'pornographic' becomes 'blue'.
'they' enclosing 'bo' is 'theboy'.
'theboy' enclosing 'blue' is 'THE BLUE BOY'.
(Other definitions for the blue boy that I've seen before include "Unhappy lad" , "Artist's subject" , "who posed for artist" , "picture" , "unhappy little chappie" .)