The girl dancing is not as heavy (7)
I believe the answer is:
lighter
'not as heavy' is the definition.
('light' can be the opposite of 'heavy' so perhaps 'lighter' is reasonable for 'not as heavy')
'the girl dancing' is the wordplay.
'dancing' indicates an anagram.
'the'+'girl'='thegirl'
'thegirl' with letters rearranged gives 'LIGHTER'.
'is' is the link.
(Other definitions for lighter that I've seen before include "Smoker's implement" , "Ship to shore boat" , "Boat for heavy loads and fire igniter" , "Barge (more likely to float?)" , "Barge; one needed by smoker" .)