Be of one mind about being in time (5)
I believe the answer is:
agree
'be of one mind' is the definition.
(two people who are of one mind are in agreement)
'about being in time' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'being in' indicates putting letters inside.
'time' becomes 'age' (an age is a period of time).
're' going into 'age' is 'AGREE'.
(Other definitions for agree that I've seen before include "Eager (anag)" , "chime?" , "Concur, acquiesce" , "'Assent, concur (5)'" , "confess?" .)