Understand coaches must ignore one overdue (9)
I believe the answer is:
translate
'understand' is the definition.
(I know that understand can be written as translate)
'coaches must ignore one overdue' is the wordplay.
'coaches' becomes 'trains' ('train' can be a synonym of 'coach').
'must ignore' is a deletion indicator.
'one' becomes 'I' (Roman numeral).
'overdue' becomes 'late' (similar in meaning).
'trains' with 'i' taken away is 'trans'.
'trans'+'late'='TRANSLATE'
(Other definitions for translate that I've seen before include "Express in a different language" , "Express (words) in another language" , "Restate in another language" , "Convert" , "Put in other words" .)