President Arthur's beginning to take steps (5)
I believe the answer is:
polka
'steps' is the definition.
(I've seen this before)
'president arthur's beginning to' is the wordplay.
'president' becomes 'Polk' (James K Polk, US president 1845-1849).
'beginning to' suggests taking the first letters.
The first letter of 'arthurs' is 'a'.
'polk'+'a'='POLKA'
'take' acts as a link.
(Other definitions for polka that I've seen before include "Dance in fast time with three steps and a hop" , "Take rapid steps" , "Czech dance" , "Dot may be associated with this" , "Lively dance (but dotty?)" .)