About 18months ago I cut down a tall conifer and I now have loads of nice long straight branches. Would it be ok to use these to build a trellis or would they still kill off any climbing plants I grow up them e.g. clematis ?
They'll not have any negative effect on anything growing up them, if that's what you mean? The only issue is that softwood will rot quickly if in contact with soil, so not ideal for the trellis supports.
A trowel in the hand is worth a thousand lost under a bush.
We cut back a very tall single conifer many years back. We left the tall straight bare stems still in the ground and didn't dig up the root.
We planted 2 clematis and ever lasting sweet pea plants and they have all happily coexisted very nicely.