Journey starts in the red and ends in the black (4)
I believe the answer is:
trek
'journey' is the definition.
(I know that trek is a more specific form of the action journey)
'starts in the red and ends in the black' is the wordplay.
'starts in' indicates taking the first letters.
'and' means one lot of letters go next to another.
'ends in' suggests the final letters.
The first letters of 'the red' is 'tr'.
The final letters of 'the black' are 'ek'.
'tr'+'ek'='TREK'
(Other definitions for trek that I've seen before include "Long difficult walk" , "Extended journey" , "Expedition or difficult trip" , "Long hard journey on foot" , "A long hard march" .)