Small cat starts to attack colossal hairy bear (7)
I believe the answer is:
stomach
'bear' is the definition.
(both can mean to endure or tolerate)
'small cat starts to attack colossal hairy' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cat' becomes 'tom' (a tom is a male cat).
'starts to' suggests taking the first letters.
The first letters of 'attack colossal hairy' is 'ach'.
's'+'tom'+'ach'='STOMACH'
(Other definitions for stomach that I've seen before include "Bag-like part of the alimentary canal" , "Courage - guts" , "Put up with" , "Abdomen" , "Internal organ" .)