Help a girl up (3)
I believe the answer is:
aid
'help' is the definition.
('aid' can be a synonym of 'help')
'a girl up' is the wordplay.
'girl' becomes 'di' (**).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'di' back-to-front is 'id'.
'a'+'id'='AID'
(Other definitions for aid that I've seen before include "Good point" , "Help or succour" , "grant" , "technique that can produce pregnancy" , "Care, assistance" .)