One sketching plans omits first rule — that's not so sensible (6)
I believe the answer is:
dafter
'not so sensible' is the definition.
(more daft or silly)
'one sketching plans omits first rule' is the wordplay.
'one sketching plans' becomes 'drafter' (someone who creates drafts or plans).
'omits' is a deletion indicator.
'first rule' becomes 'r' (1st letter of 'rule').
'drafter' with 'r' taken away is 'DAFTER'.
'that's' is the link.
(Other definitions for dafter that I've seen before include "crazier" , "More foolish!" .)