Heading for stairs, step in room (5)
I believe the answer is:
space
'room' is the definition.
(thesaurus)
'heading for stairs step' is the wordplay.
'heading for' indicates taking the first letters.
'step' becomes 'pace' ('pace' can be a synonym of 'step').
The first letter of 'stairs' is 's'.
's'+'pace'='SPACE'
'in' is the link.
(Other definitions for space that I've seen before include "Free area" , ""The final frontier"" , "sci-fi theme" , "Gap; --- Oddity (Bowie)" , "Interstellar vacuum" .)