Having meant to turn first left, be sorry about it (6)
I believe the answer is:
lament
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'having meant to turn first left be sorry' is the wordplay.
I cannot really understand how this works, but
an anagram of 'meant' is 'ament' which is located in the answer.
'left' could be 'l' (common abbreviation) and 'l' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'about' is the link.
Can you help me to learn more?
(Other definitions for lament that I've seen before include "Mourn for; deplore" , "Song expressing grief" , "Keen" , "Dart-throwing line" , "Sad song" .)