Fluster in the end a little bit and struggle to manage (3,4)
I believe the answer is:
rat race
'struggle to manage' is the definition.
(the struggle to manage in life)
'fluster in the end a little bit' is the wordplay.
'in the end' indicates one should take the final letters.
'little bit' becomes 'trace' (a trace is a small amount).
The last letter of 'fluster' is 'r'.
'r'+'a'+'trace'='RAT-RACE'
'and' acts as a link.
(Other definitions for rat race that I've seen before include "Life's competition" , "Fiercely competitive struggle in business" , "tough competition" , "Continual routine of highly competitive activity" , "unhealthy competition" .)