Tons raced to take a seat for crossing (7)
I believe the answer is:
transit
'crossing' is the definition.
(thesaurus)
'tons raced to take a seat' is the wordplay.
'tons' becomes 't'.
'raced' becomes 'ran' ('run' can be a synonym of 'race').
'to take a seat' becomes 'sit' (I've seen this before).
't'+'ran'+'sit'='TRANSIT'
'for' is the link.
(Other definitions for transit that I've seen before include "In migration" , "Pass across" , "Act of passing across" , "Passage of a planet across the sun's disc" , "To pass through/across" .)