Careless delay about the start of day can be fatal (6)
I believe the answer is:
deadly
'be fatal' is the definition.
(similar in meaning)
'careless delay about the start of day' is the wordplay.
'careless' is an anagram indicator.
'about' indicates putting letters inside.
'the start of' suggests taking the first letters.
The first letter of 'day' is 'd'.
'delay' anagrammed gives 'dealy'.
'dealy' going around 'd' is 'DEADLY'.
'can' acts as a link.
(Other definitions for deadly that I've seen before include "Lethal or mortal" , "sniper aspires to be" , "No small bore" , "Belladonna" , "There are seven such sins" .)