A little sister I called Heather (5)
I believe the answer is:
erica
'heather' is the definition.
(the plant genus Erica includes heather plants)
'a little sister i called' is the wordplay.
'a little' indicates a hidden word (a little bit of the clue).
'ERICA' is hidden in the letters of 'sister i called'.
(Other definitions for erica that I've seen before include "Woman" , "Moorland plant, girl's name" , "Female" , "Girl's name - I care (anag)" , "I care about Heather, oddly" .)