Enigmatic Variations No.1167 – Warning by Samuel

A somewhat unusual set-up: across clues are given in pairs, and the method of entry for four across rows is thematic (whatever that might turn out to mean), with no definition given.  Another row is unclued, and must be filled with ‘the reason for the WARNING’.  The down clues, meanwhile, are not idle – extra letters from their solutions give ‘the subject of the WARNING’.

I appreciated that the down clues that did not give extra letters separated the thematic phrase into words, giving GREEKS BEARING GIFTS.

The most famous Greek-borne gift was the TROJAN HORSE, which is to be written in the third (unclued) row.

The method of the four other thematic rows is obvious enough once one is solved.  The two answers are a Greek and a gift (in whatever order), and the latter is to be inserted in the former.  For example, the first row reads AGAMEMSOPNON.

Notation
(xxx) = definition
[xxx] = (anagram/homophone/container/etc.) indicator
XXX* = anagram
< = reversal
GALAS = additional letter

Please post a comment if the explanations are not clear.