People get old, hiding growing resentment — like all the animals (9)
I believe the answer is:
menagerie
'the animals' is the definition.
(I've seen this before)
'people get old hiding growing resentment like all' is the wordplay.
I cannot really see how this works, but
'people' could be 'men' (men can refer to humanity as a whole) and 'men' is present in the answer.
'get old' could be 'age' (synonyms) and 'age' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for menagerie that I've seen before include "Animals go in here" , "place to keep animals" , "Animal exhibition" , "Collection of live animals for study or display" , "A lot of wild animals" .)