The question of a trip being about five hundred pounds! (6)
I believe the answer is:
riddle
'the question' is the definition.
(riddle is a kind of question)
'a trip being about five hundred pounds' is the wordplay.
'a trip' becomes 'ride' (eg a trip in a car).
'being about' is an insertion indicator.
'five hundred' becomes 'D' (Roman numeral for 500).
'pounds' becomes 'L' (L can represent the monetary pound).
'd'+'l'='dl'
'ride' enclosing 'dl' is 'RIDDLE'.
'of' acts as a link.
(Other definitions for riddle that I've seen before include "unsolved problem" , "Puzzling question - sieve" , "Ingenious question" , "Sieve or conundrum" , "Question to test ingenuity, something puzzling" .)