Cruel keeping some chipmunk indoors (6)
I believe the answer is:
unkind
'cruel' is the definition.
(synonyms)
'some chipmunk indoors' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'UNKIND' is hidden in the letters of 'chipmunk indoors'.
'keeping' is the link.
(Other definitions for unkind that I've seen before include "Uncaring, nasty" , "beastly to others?" , "Cold-hearted" , "Mean, pitiless" , "Not gentle or considerate" .)