Enigmatic Variations No.1191 – Briefing Received by Gaston

Prior to this puzzle, there have been a couple of weeks where the EVs had misspellings in the clues’ definitions, which I tend to find rather hard.  I’m therefore heartened to see in the preamble that this one has extra letters in the wordplay.  There’s some highlighting too, which can sometimes give the puzzled puzzler an extra hint.

As it turns out, I needn’t have worried.  The first clues I solve are the four twelve-letter ones forming a square in the centre of the grid.  From there, it’s a rapid and pleasurable grid fill.

The BRIEFING RECEIVED – given by extra letters from across clues – is PUT I BEFORE E EXCEPT AFTER C.  The title consists of two words that ‘obey’ each of its ‘two parts’ (in the wording of the preamble).

The ‘excuse for disregarding’ it – given by extra letters from down clues – is EXCEPTIONS PROVE THE RULE.  In line with this, the five normally clued across entries – KEISTER, MADEIRA, SURFEIT, CODEINE, and SEINERS – disobey ‘one part’ by putting E before I, even though there’s no preceding C, while the two normally clued down entries – SCIENCE and SPECIES – disobey the ‘other part’ by putting I before E even though there is a preceding C.

These seven grid entries, which are to be highlighted, form an I in the NW corner of the grid and an E in the SE corner (thus ‘putting I before E’).

I’m not sure that I have parsed 29 across correctly.

Notation

(xxx) = definition
[xxx] = (anagram/homophone/container/etc.) indicator
XXX* = anagram
< = reversal
PROP = extra letter

Please post a comment if the explanations are not clear.