Long periods entertaining Northern girl (5)
I believe the answer is:
agnes
'girl' is the definition.
'long periods entertaining northern' is the wordplay.
'long periods' becomes 'ages' (I've seen this before**).
'entertaining' means one lot of letters goes inside another.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'ages' enclosing 'n' is 'AGNES'.
(Other definitions for agnes that I've seen before include "Woman" , "Miss Grey" , "Tube for cables" , "The Eve of St. - (Keats)" , "Scandinavian singer" .)