Short fit relative (7)
I believe the answer is:
grandma
'short fit relative' is the definition.
'grandma' can be an answer for 'relative' (grandma is a kind of relative). I'm unsure of the 'short fit' bit.
'fit relative' is the wordplay.
'fit' becomes 'grand' (I can't explain this - if you can you should believe this answer much more).
'relative' becomes 'ma' (ma is a kind of relative).
'grand'+'ma'='GRANDMA'
Can you help me to learn more?
(Other definitions for grandma that I've seen before include "family member" , "standing before Moses in painting" , "Drag man out to meet the old lady" , "Rang dam about a relative" , "Drag man to his relative" .)