Bridge piles are up (4)
I believe the answer is:
span
'bridge' is the definition.
(I know that bridge can be written as span)
'piles are up' is the wordplay.
'piles' becomes 'naps' (I've seen this in another clue).
'are up' says the letters should be written backwards (in down clue: letters go upwards).
'naps' reversed gives 'SPAN'.
(Other definitions for span that I've seen before include "Distance between bridge abutments" , "Part of bridge, nine inches" , "Measurement taken from an extended hand" , "Extent of aircraft's wing" , "Extent of bridge or life" .)