Saw girl, huge, being cut in half (5)
I believe the answer is:
adage
'saw' is the definition.
('adage' can be a synonym of 'saw')
'girl huge being cut in half' is the wordplay.
'girl' becomes 'ada' (**).
'being cut in half' indicates taking half.
'huge' with half the letters taken is 'ge'.
'ada'+'ge'='ADAGE'
(Other definitions for adage that I've seen before include "Saying, saw" , "Slogan" , "Old saw" , "Axiom" , "say it again and again!" .)