Undertaking quest as knight? Not entirely (4)
I believe the answer is:
task
'undertaking' is the definition.
(I know that undertaking can be written as task)
'quest as knight? not entirely' is the wordplay.
'not entirely' indicates a hidden word.
'TASK' can be found hidden inside 'quest as knight'.
(Other definitions for task that I've seen before include "exercise" , "Job of work" , "commission" , "Burden with work" , "Set piece of work" .)