Tip off a Romeo to get into woman's skirt (4)
'tip off' is the definition.
(I've seen this before)
'a romeo to get into woman's skirt' is the wordplay.
'romeo' becomes 'R' (phonetic alphabet: alpha, bravo, charlie etc.).
'to get into' indicates putting letters inside.
'skirt' means to remove the middle letters (skirt can mean the edge or outside of something).
'woman' with its centre taken out is 'wn'.
'ar' going inside 'wn' is 'WARN'.
(Other definitions for warn that I've seen before include "Notify in advance of danger", "Give a caution to", "Admonish, caution", "Tip off", "Sound the alarm".)