Being late is a bit unsettling for a slow starter (6)
I believe the answer is:
sunset
'a slow starter' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'being late is a bit unsettling for' is the wordplay.
'being late' indicates one should take the final letters (I've seen 'latest of' mean this).
'a bit' says to take the initial letters (I've seen 'a bit of' mean this).
'for' becomes 'nset' (I can't justify this - if you can you should give a lot more credence to this answer).
The final letter of 'is' is 's'.
The first letter of 'unsettling' is 'u'.
's'+'u'+'nset'='SUNSET'
Can you help me to learn more?
(Other definitions for sunset that I've seen before include "star not in the ascendant at this time" , "When the light begins to fall below the horizon" , "Musical from film about Hollywood" , "End of the day in terms of light" , "After which day becomes night" .)