Expedition fails finally in dreadful heat (5)
I believe the answer is:
haste
'expedition' is the definition.
(expedition is expediting, doing something quickly)
'fails finally in dreadful heat' is the wordplay.
'finally' says to take the final letters.
'in' means one lot of letters goes inside another.
'dreadful' indicates anagramming the letters.
The final letter of 'fails' is 's'.
'heat' is an anagram of 'hate'.
's' put within 'hate' is 'HASTE'.
(Other definitions for haste that I've seen before include "Rash speed" , "More of this proverbially slows you down" , "The more of it, the less speed sometimes" , "''Marry in . . . . ., repent at leisure''" , "Inconsiderate speed" .)