Fit to run across (4)
I believe the answer is:
meet
I believe this is a double definition.
'fit' is the first definition.
(meeting is a kind of fitting)
'run across' is the second definition.
('meet' can be a synonym of 'run across')
'to' is the link.
(Other definitions for meet that I've seen before include "Touch; suitable (arch.)" , "Attend" , "Converge" , "Encounter - suitable - pay" , "Countryside assembly now perhaps not legal" .)