A fiercely competitive life run into a crater! (3,4)
I believe the answer is:
rat race
'a fiercely competitive life run' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'a crater' is the wordplay.
'a crater' anagrams to 'RAT RACE'.
However, I'm unsure how this is indicated.
'into' acts as a link.
Can you help me to learn more?
(Other definitions for rat race that I've seen before include "Daily struggle to get on" , "Competitive struggle for wealth or power" , "constant effort to get ahead" , "life's futile struggle" , "Grim struggle" .)