Key singer holding a run - it may go up or down (9)
I believe the answer is:
escalator
'it may go up or down' is the definition.
'key singer holding a run' is the wordplay.
'key' becomes 'esc' ('Escape' key on a computer keyboard**).
'singer' becomes 'alto' (alto is a kind of singer).
'holding' is an insertion indicator.
'run' becomes 'r' (cricket abbreviation).
'alto' enclosing 'a' is 'alato'.
'esc'+'alato'+'r'='ESCALATOR'
(Other definitions for escalator that I've seen before include "it moves up and down" , "effortless steps" , "nonstop flight" , "Moving stairway" , "Mechanised staircase" .)