Alert lord managed a difficult task (4,5)
I believe the answer is:
tall order
'a difficult task' is the definition.
(I've seen this before)
'alert lord managed' is the wordplay.
'managed' indicates anagramming the letters.
'alert'+'lord'='alertlord'
'alertlord' with letters rearranged gives 'TALL ORDER'.
(Other definitions for tall order that I've seen before include "for brotherhood of high standing" , "Challenge" , "Unreasonable demand" , "Lofty command - that's a difficult task" , "Difficult demand" .)