Gertrude —, London-born garden designer who died in 1932 (6)
I believe the answer is:
jekyll
(Other definitions for jekyll that I've seen before include "Hyde's counterpart" , "Famous novel 'Dr ...... and Mr Hyde'(6) ()" , "Robert Louis Stevenson novel 'Dr . . . . . . and Mr Hyde'" , "The good personality (see 18)" , "Famous novel' Dr......and Mr Hyde'" .)