It was a charmless Christmas for Josephine, in short, but she'll get some support from these. (6)
I believe the answer is:
joists
'short but she'll get some support from these' is the definition.
The definition suggests an adverb but the answer is not.
'it was a charmless christmas for josephine' is the wordplay.
I cannot quite see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
'was' could be 'is' (being is a kind of being) and 'is' is found in the answer.
'josephine' could be 'jo' (I've seen this in other clues) and 'jo' is present in the answer.
A single letter 's' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
Can you help me to learn more?
(Other definitions for joists that I've seen before include "Beams for floor or roof" , "Supporting beams in a floor or ceiling" .)