Set step across line (5)
I believe the answer is:
place
'set' is the definition.
(I know that set can be written as place)
'step across line' is the wordplay.
'step' becomes 'pace' (synonyms).
'across' means one lot of letters goes inside another.
'line' becomes 'l' (used when specifying particular lines from a poem).
'pace' enclosing 'l' is 'PLACE'.
(Other definitions for place that I've seen before include "Locate" , "Spot" , "Situation" , "Site" , "Venue" .)