Clear account, pretty short (6)
I believe the answer is:
acquit
'clear' is the definition.
('acquit' can be a synonym of 'clear')
'account pretty short' is the wordplay.
'account' becomes 'ac'.
'pretty' becomes 'quite' ('quite' can be a synonym of 'pretty').
'short' means to remove the last letter.
'quite' with its last letter taken off is 'quit'.
'ac'+'quit'='ACQUIT'
(Other definitions for acquit that I've seen before include "Group of families" , "Let off" , "Declare to be innocent in court" , "Declare to be innocent of a crime" , "Free (from charge)" .)