Relation making excursion heading off (4)
I believe the answer is:
aunt
'relation' is the definition.
(I've seen this before)
'excursion heading off' is the wordplay.
'excursion' becomes 'jaunt' ('jaunt' can be a synonym of 'excursion').
'heading off' means to remove the first letter.
'jaunt' with its first letter taken away is 'AUNT'.
'making' acts as a link.
(Other definitions for aunt that I've seen before include "Parent's female sibling" , "Close relative" , "Charley's in play?" , "Charley's ___ , farce staged in London in 1892, revived and adapted ever since" , "Parent's sister" .)