Bear cat in bag hunter's opening (7)
I believe the answer is:
stomach
'bear' is the definition.
('stomach' can be a synonym of 'bear')
'cat in bag hunter's opening' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'in' is an insertion indicator.
'bag' becomes 'sac' (I've seen this before).
'opening' says to take the initial letters.
The initial letter of 'hunters' is 'h'.
'tom' put inside 'sac' is 'stomac'.
'stomac'+'h'='STOMACH'
(Other definitions for stomach that I've seen before include "Bag-like part of the alimentary canal" , "Endure, accept" , "Inside" , "Abdomen" , "stand" .)