Sister with new uniform, head of nurses (3)
I believe the answer is:
nun
'sister' is the definition.
(sister is a term of address for a nun)
'new uniform head of nurses' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'uniform' becomes 'u' (phonetic alphabet: alpha, bravo, charlie etc.).
'head of' says to take the initial letters.
The first letter of 'nurses' is 'n'.
'n'+'u'+'n'='NUN'
'with' acts as a link.
(Other definitions for nun that I've seen before include "Member of convent" , "Woman in religious order" , "One who's taken vows" , "Woman in religious community" , "Domestic fancy pigeon" .)