Runner's day cutting across the Atlantic (5)
I believe the answer is:
indus
'runner's' is the definition.
(Indian river)
'day cutting across the atlantic' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'cutting' indicates putting letters inside (some letters cut their way into a word).
'across the atlantic' becomes 'in US' (in the United States).
'd' put into 'inus' is 'INDUS'.
(Other definitions for indus that I've seen before include "running water" , "banker" , "River of Southern Asia" , "Asian runner" , "Largest river of Pakistan" .)