This is the first introduction of a pronoun – "you." So we know the poem will either be addressed to this "you" in an "I" and "you" relationship; or the "you" will become part of a "we."
"Flesh" isn't a completely strange or disgusting way to describe fruit, but it does make us think of actual animal flesh. Especially coming right after "clot" in line 3. Are you thinking what we're thinking – flesh and blood?
And we're introduced to a third sense, taste.
Like thickened wine: summer's blood was in it
Again, we're reminded of the texture. This berry's juice is thick.
And again, we've got a reference (especially direct this time) to blood. But now we see that it's not going to get too gross because we're just talking about the figurative – not actual – blood of summer.
In the first six lines we have flesh, blood, and wine. Heaney was raised Catholic and we're seeing a little of the Eucharist here! The Eucharist is a Christian ceremony where bread and wine, representing Jesus Christ's blood and body, are consumed to commemorate the Last Supper, which was held the night before Jesus was killed.
Leaving stains upon the tongue and lust for
The berry juice stains the tongue when eaten.
It's getting sexy in here – tongue and lust. Where the line is broken, we're left wondering: lust for what?
Again, we're indirectly reminded of taste. So far Heaney's made use of three out of our five senses.