Female monarch one having vacated Clytha (5)
I believe the answer is:
erica
'female' is the definition.
(I've seen this before)
'monarch one having vacated clytha' is the wordplay.
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'one' becomes 'i' (Roman numeral).
'having' says to put letters next to each other.
'vacated' means to remove the middle letters.
'clytha' with its centre removed is 'ca'.
'er'+'i'+'ca'='ERICA'
(Other definitions for erica that I've seen before include "Heath genus; girl's name" , "Heather, for instance" , "Girl's name - I care (anag)" , "hardy shrub" , "Shrub genus; girl's name" .)