Blur challenged to cover rock (5)
I believe the answer is:
lurch
'rock' is the definition.
(thesaurus)
'blur challenged to cover' is the wordplay.
'to cover' indicates a hidden word (I've seen 'covered' mean this (the answer is covered up by the clue)).
'LURCH' can be found hidden inside 'blur challenged'.
(Other definitions for lurch that I've seen before include "Sudden unsteady movement" , "Walk with an unsteady uneven gait" , "Sudden unsteady move" , "Move forward suddenly and unsteadily" , "Move abruptly to one side" .)