Drunken tramp in US showing small wet, round bottom (7)

'drunken tramp in us' is the definition.

'small wet round bottom' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'round' says the letters should be written backwards.
'bottom' becomes 'bum' ('bum' can be a synonym of 'bottom').
'wet' in reverse letter order is 'tew'.
's'+'tew'+'bum'='STEWBUM'

'showing' is the link.