Young woman in regalia (3)
I believe the answer is:
gal
'young woman' is the definition.
(synonyms)
'in regalia' is the wordplay.
'in' indicates a hidden word.
'GAL' is hidden within 'regalia'.
(Other definitions for gal that I've seen before include "measure of acceleration?" , "Young woman (colloq.)" , "Girl from American west" , "Lady" , "US female" .)