Jack's companion, a tumbler (4)
I believe the answer is:
jill
This clue could be a double definition.
'jack's companion' is the first definition.
(I've seen this before)
'a tumbler' is the second definition.
Both the definition and answer are singular nouns.
Maybe they are linked in a way I don't understand?
Can you help me to learn more?
(Other definitions for jill that I've seen before include "Jack's pet" , "Female ferret" , "Jack's nursery rhyme girl companion" .)