Growths initially found upsetting New Guinea islander (5)
I believe the answer is:
fungi
'growths' is the definition.
'initially found upsetting new guinea islander' is the wordplay.
'initially' indicates taking the first letters.
The first letters of 'found upsetting new guinea islander' is 'FUNGI'.
(Other definitions for fungi that I've seen before include "Eg, toadstools" , "Yeasts, moulds, mushrooms etc" , "Magic mushrooms perhaps" , "Group with some killers" , "Spore-producing organisms" .)