Hi, Whistler.
19a: definition is the first two words. 'Girl' is a common synonym.
21d: definition is the last word, If you know French, it will help you. It was one of the favourite words of existentialist novelists.
22d: the definition is the last word but it's not the everyday meaning. The wordplay is speculation followed by Delia with subtractions.
25d: my least favourite clue. The synonym for 'important person' is really obscure. The best bet is to think of synonyms for 'mighty' that would fit and parse it from there.