Twice the helium and lithium unknown in molecule (6,5)
I believe the answer is:
double helix
'molecule' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see an association between them that I can't see?
'twice the helium and lithium unknown' is the wordplay.
'twice' becomes 'double' (synonyms).
'the helium' becomes 'He' (chemical symbol).
'and' means one lot of letters go next to another.
'lithium' becomes 'Li' (chemical symbol).
'unknown' becomes 'x' (algebra).
'double'+'he'+'li'+'x'='DOUBLE HELIX'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for double helix that I've seen before include "Shape of a DNA molecule" , "Structure of DNA" , "Pair of spirals about a common axis" .)