Reckonin'

Published in the Sunday Telegraph on 31 July 2016 as Enigmatic Variations Puzzle no.1238, Reckonin' by Eclogue

This title is taken from the last verse of the (tongue in cheek?) poem "Oh, I Wish I'd Looked After My Teeth" by Pam Ayres in which she bemoans the parlous state of her teeth due to casual neglect throughout her younger years..

Extra letters generate a couplet from the poem and the author's name.  The title itself appears in the grid in a mouth-like formation to be highlighted

but involves four clashes which must be blacked in to resemble bad teeth.  The clashes involve several letters each and theses can be resolved to spell different teeth situated in the "mouth" in approximately their correct positions.