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Seventeen words carry N second and D third, and every one of them opens with a vowel.
I opens six of the seventeen, U opens six and E opens five. There is nothing else, so the first box is a three way choice every single time.
UNDER and UNDID use un in its ordinary sense, but UNDUE means not due and UNDUG means not dug. INDEX and INDRI are not prefixed at all, they simply begin with those letters.
So you cannot reason from the opening. ENDED and ENDER come from the verb end, while ENDOW and ENDUE come from Latin roots that have nothing to do with ending.
W takes four of the seventeen, level with E. ENDOW, INDOW, ENDEW and INDEW are variant spellings clustered around the same two words.
E fills the fourth box in seven of the seventeen, with U on four. Between them the fourth letter is nearly settled before you begin.
UNDER tests the commonest opening and the R ending. INDEX is worth trying only if you suspect the X.
A large lemur from Madagascar, known for its loud wailing call. It is the one entry here built from neither a prefix nor a plain verb.
They are two spellings of the same verb, meaning to endow or clothe with a quality. Both are valid and neither is common.
Wavy, in heraldry, describing a line with an undulating form. It is specialist vocabulary from coats of arms.