DNA half (5)
I believe the answer is:
helix
'dna half' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for helix that I've seen before include "'Spiral structure, DNA is a double one (5)'" , "snail" , "One of two DNA spirals" , "that goes round and round and up and down" , "Three-dimensional object shaped like a corkscrew" .)