King, tense over guards on journey (4)
I believe the answer is:
trek
'journey' is the definition.
(I know that trek is a more specific form of the action journey)
'king tense over guards on' is the wordplay.
'king' becomes 'K' (chess abbreviation).
'tense' becomes 't' (abbreviation used in many dictionaries).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'guards' means one lot of letters goes inside another (inserted letters are guarded).
'on' becomes 're' ('on' can be mean 'with reference to').
'k' put after 't' is 'tk'.
'tk' going around 're' is 'TREK'.
(Other definitions for trek that I've seen before include "Long and difficult trip" , "A long arduous walk" , "Long difficult walk" , "Expedition or difficult trip" , "Slog" .)