Try hard flying over a dune (9)
I believe the answer is:
endeavour
'try hard' is the definition.
(I've seen this before)
'flying over a dune' is the wordplay.
'flying' indicates anagramming the letters.
'over'+'a'+'dune'='overadune'
'overadune' with letters rearranged gives 'ENDEAVOUR'.
(Other definitions for endeavour that I've seen before include "Strive - Inspector Morse's first name" , "Try - Cook's ship" , "famous ship" , "Captain Cook's first ship - DCI Morse's first name" , "Try - Captain Cook's vessel" .)