Subject to series of profound errors (5)
I believe the answer is:
under
'subject to' is the definition.
(being subject to a law is being under it)
'series of profound errors' is the wordplay.
'series of' indicates the answer is hidden within the clue (a series of letters from the clue).
'UNDER' can be found hidden inside 'profound errors'.
(Other definitions for under that I've seen before include "Being short" , "Subordinate to; planted with" , "Lower than" , "See 16 Down" , "Managed by" .)