American upper class into new, clipped Lambeth Walk (8)
I believe the answer is:
ambulate
'walk' is the definition.
(I've seen this before)
'american upper class into new clipped lambeth' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'upper class' becomes 'u'.
'into' indicates putting letters inside.
'new' is an anagram indicator.
'clipped' means to remove the last letter.
'lambeth' with its last letter taken off is 'lambet'.
'lambet' is an anagram of 'mblate'.
'u' put inside 'mblate' is 'mbulate'.
'a'+'mbulate'='AMBULATE'
(Other definitions for ambulate that I've seen before include "Walk (technical)" , "Go for a stroll" .)