Invariably a woman has a Romeo (4)
I believe the answer is:
ever
'invariably' is the definition.
(I've seen this in another clue)
'a woman has a romeo' is the wordplay.
'a woman' becomes 'eve' (Eve is an example).
'has' is a charade indicator (letters next to each other).
'a romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'eve'+'r'='EVER'
(Other definitions for ever that I've seen before include "Continually" , "On any occasion" , "Perpetually" , "At all times, always" , "In perpetuity" .)