Theatre sniffy about content being put in (6)
I believe the answer is:
insert
'being put in' is the definition.
(I've seen this in another clue)
'theatre sniffy about content' is the wordplay.
'content' says the answer is hidden in the clue (letters contained within the clue).
'about' shows that the letters should be reversed in order.
'tresni' is hidden within 'theatre sniffy'.
'tresni' in reverse letter order is 'INSERT'.
(Other definitions for insert that I've seen before include "Put something inside something else" , "Place within, pages of a publication say" , "Introduce, put in" , "Place in between other things" , "One might be seen" .)