Ann becomes engaged to a lean historian (8)
I believe the answer is:
annalist
'historian' is the definition.
(annalist is a kind of historian)
'ann becomes engaged to a lean' is the wordplay.
I cannot really understand how this works, but
an anagram of 'ann' is 'nna' which is located in the answer.
'a' is within the answer.
'lean' could be 'list' (both can mean to tilt) and 'list' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for annalist that I've seen before include "One making records" , "Historian" , "Person creating records" , "one for keeping records" , "Chronicler" .)