Cat's in the bag — start to hunt bear (7)
I believe the answer is:
stomach
'bear' is the definition.
(synonyms)
'cat's in the bag start to hunt' is the wordplay.
'cat' becomes 'tom' (a tom is a male cat).
'in' indicates putting letters inside.
'the bag' becomes 'sac' (I've seen this before).
'start to' says to take the initial letters.
The initial letter of 'hunt' is 'h'.
'tom' placed inside 'sac' is 'stomac'.
'stomac'+'h'='STOMACH'
(Other definitions for stomach that I've seen before include "Digestive organ" , "Put up with" , "Breadbasket (informal)" , "Tummy" , "Tolerate; appetite" .)