Uncharacteristically, I'm in a very short skirt (4)
I believe the answer is:
mini
'a very short skirt' is the definition.
(I've seen this before)
'uncharacteristically i'm in' is the wordplay.
'uncharacteristically' indicates anagramming the letters (uncharacteristically means different from usual).
'im'+'in'='imin'
'imin' anagrammed gives 'MINI'.
(Other definitions for mini that I've seen before include "Cult British car" , "type of garment" , "Once fashionable skirt" , "(Car etc) of small size" , "Small (of car, skirt, disk,bar or series)" .)