Strange sailor up for dance (5)
I believe the answer is:
'dance' is the definition.
(rumba is a kind of dance)
'strange sailor up' is the wordplay.
'strange' becomes 'rum' (synonyms).
'sailor' becomes 'ab' (abbreviation for able seaman).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'ab' written backwards gives 'ba'.
'for' acts as a link.
(Other definitions for rumba that I've seen before include "What to perform" , "Lively Cuban dance" , "Latin beat" , "Style of dance" , "one's on the floor for it" .)