Ladder, or short part of one (3)
I believe the answer is:
run
'ladder' is the definition.
('run' can be a synonym of 'ladder')
'or short part of one' is the wordplay.
'short' means to remove the first letter.
'part of one' becomes 'un' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'or' with its initial letter taken off is 'r'.
'r'+'un'='RUN'
(Other definitions for run that I've seen before include "Administer" , "Organize" , "Manage (eg, a company)" , "Flow quickly" , "Race, enclosure" .)