She nags restlessly about an early start (5)
I believe the answer is:
agnes
'she' is the definition.
'nags restlessly about an early start' is the wordplay.
'restlessly' is an anagram indicator (I've seen this in other clues).
'about' means one lot of letters goes inside another.
'start' says to take the initial letters.
The initial letter of 'early' is 'e'.
'nags' anagrammed gives 'agns'.
'agns' enclosing 'e' is 'AGNES'.
(Other definitions for agnes that I've seen before include "Woman" , "Girl's name" , "Lady" , "girl" , "lass" .)