Girl losing her head over slang (5)
I believe the answer is:
lingo
'slang' is the definition.
(lingo can mean slang or jargon)
'girl losing her head over' is the wordplay.
'girl' becomes 'lin' ().
'losing her head' means to remove the first letter.
'over' becomes 'ago' (both can mean in the past).
'ago' with its first letter removed is 'go'.
'lin'+'go'='LINGO'
(Other definitions for lingo that I've seen before include "Language (informally)" , "special words" , "Foreign language or dialect: jargon" , "local dialect" , "speech" .)