Maxim's almost bottom up (5)
I believe the answer is:
motto
'maxim's' is the definition.
(I've seen this in another clue)
'almost bottom up' is the wordplay.
'almost' means to remove the first letter.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'bottom' with its initial letter taken off is 'ottom'.
'ottom' reversed gives 'MOTTO'.
(Other definitions for motto that I've seen before include "It may be proverbial" , "Cracker slip" , "(Family) maxim" , "Brief guiding rule" , "statement of belief" .)