Be arriving for fitting (8)
I believe the answer is:
becoming
'fitting' is the definition.
(thesaurus)
'be arriving' is the wordplay.
'arriving' becomes 'coming' ('come' can be a synonym of 'arrive').
'be'+'coming'='BECOMING'
'for' acts as a link.
(Other definitions for becoming that I've seen before include "Growing" , "Developing into" , "Appropriate" , "Undergoing a change" , "Befitting or enhancing" .)