New girl with head of year - a children's nurse (5)
I believe the answer is:
nanny
'a children's nurse' is the definition.
(I've seen this before)
'new girl with head of year' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'girl' becomes 'Ann' (woman's name).
'with head of' says to take the initial letters.
The first letter of 'year' is 'y'.
'n'+'ann'+'y'='NANNY'
(Other definitions for nanny that I've seen before include "butter" , "Carer for children" , "One looking after children" , "Child minder - goat - grandma" , "She cares for kids" .)