'Head off, bottom up' as a slogan (5)
I believe the answer is:
motto
'a slogan' is the definition.
('motto' can be a synonym of 'slogan')
'head off bottom up' is the wordplay.
'head off' means to remove the first letter.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'bottom' with its first letter removed is 'ottom'.
'ottom' backwards is 'MOTTO'.
'as' is the link.
(Other definitions for motto that I've seen before include "Catchphrase" , "Part of coat of arms" , "Saw" , "Short sentence expressing an ideal" , "Short sentence adopted as rule of conduct" .)