Disreputable, extremely short, wasn't standing up straight (6)
I believe the answer is:
louche
'disreputable' is the definition.
('louche' can be similar in meaning to 'disreputable')
'extremely short wasn't standing up straight' is the wordplay.
'extremely short' means to look at the middle letters (letters on the extremes of the word are removed).
'wasn't standing up straight' becomes 'slouched'.
The middle of 'slouched' is 'LOUCHE'.
(Other definitions for louche that I've seen before include "Rakishly disreputable" , "Seedy" , "Appealingly disreputable" , "Disreputable in a rakish sort of way" , "Ignominious" .)