Chat a girl up? (3)
I believe the answer is:
yak
'chat' is the definition.
(thesaurus)
'a girl up?' is the wordplay.
'a girl' becomes 'kay'.
'up?' says the letters should be written backwards (in down clue: letters go upwards).
'kay' backwards is 'YAK'.
(Other definitions for yak that I've seen before include "Tibetan beast; chatter" , "A long-haired Asian ox" , "Domesticated ox" , "Persistent talk" , "Species of ox" .)