Montana's capital (6)
I believe the answer is:
'montana's capital' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for helena that I've seen before include "Hermia's friend, rival (MND)" , "Could be Rubinstein" , "Island saint in the South Atlantic" , "St ---, where Napoleon died" , "Girl's name - US state capital" .)