Ambassador has lie-ins regularly with a woman (6)
I believe the answer is:
helena
'woman' is the definition.
'ambassador has lie-ins regularly with a' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'has' says to put letters next to each other.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'with' says to put letters next to each other.
'lie'+'ins'='lieins'
The alternating letters of 'lieins' are 'len'.
'he'+'len'+'a'='HELENA'
(Other definitions for helena that I've seen before include "Could be Rubinstein" , "- - Bonham-Carter, actress" , "Hermia's friend, rival (MND)" , "-- Bonham Carter, actress" , "dreamy girl" .)