Otherwise girl has one missing (4)
I believe the answer is:
else
'otherwise' is the definition.
(synonyms)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for else that I've seen before include "Adverb meaning if not, otherwise" , "In addition, besides" , "As an alternative" , "Alternatively" , "Eels (anag)" .)