Historian girl is entertained by lieutenant (8)
I believe the answer is:
annalist
'historian' is the definition.
(annalist is a kind of historian)
'girl is entertained by lieutenant' is the wordplay.
'girl' becomes 'anna' ().
'entertained by' is an insertion indicator (inserted letters are hosted or entertained).
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'is' placed inside 'lt' is 'list'.
'anna'+'list'='ANNALIST'
(Other definitions for annalist that I've seen before include "One who makes records" , "she makes record" , "recorder" , "Chronicler" , "Person creating records" .)