Slogan backing bottomless clothes (5)
I believe the answer is:
motto
'slogan' is the definition.
('motto' can be a synonym of 'slogan')
'backing bottomless clothes' is the wordplay.
'backing' shows that the letters should be reversed in order.
'clothes' indicates the answer is hidden within the clue (the letters clothe the hidden word).
'ottom' is hidden in the letters of 'bottomless'.
'ottom' backwards is 'MOTTO'.
(Other definitions for motto that I've seen before include "Message in cracker" , "Saw" , "(Family) maxim" , "Recurring musical phrase" , "Words on heraldic crest" .)