Some pretty pessimistic people (5)
I believe the answer is:
types
'people' is the definition.
(type is a kind of person)
'some pretty pessimistic' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'TYPES' is hidden in the letters of 'pretty pessimistic'.
(Other definitions for types that I've seen before include "Sorts; kinds" , "Categories having common characteristics" , "Classes or groups" , "Characters" , "Sorts - varieties" .)