Quarrel about length of plane journey (6)
I believe the answer is:
flight
'plane journey' is the definition.
(I've seen this before)
'quarrel about length' is the wordplay.
'quarrel' becomes 'fight' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'length' becomes 'l' (abbreviation in geometry).
'fight' going around 'l' is 'FLIGHT'.
'of' acts as a link.
(Other definitions for flight that I've seen before include "the vanquished may take?" , "A series of stairs" , "Stairs; escape" , "Set of stairs" , "Journey by air" .)