New light fitting- Spare earth wire from ceiling

An earth wire was presumably put there for a reason, which was probably not to be a 'spare'. It may have been that there was a break somewhere which has been overcome by someone adding a separate 'earth' wire. With no other information, I would connect this earth wire to all the others.

if the light fitting you want to install has metal parts and an earth lead its likely to be what's knows as a Class 1 fitting which requires an earth for safety - you will have to check the earth loop impendence at the ceiling supply. this requires a loop tester or a continuity tester and is a job for some one who is competent. There's no such thing as spare earths - generally they are always connected together its not something that's as simple or straightforward as it appears to be. it would be possible to reduce the safety of parts of the lighting installation if this work is carried out incorrectly

Even if your fitting does not require the ‘earth’ don’t poke it back into the ceiling. Bring the ‘earth’ into the fitting with the other wires, then use your choc block on it. You might not need it now but someone will later and even if you don’t if the property ever requires an electrical inspection, missing CPC ‘earths’ would be a problem.

Robin's response is most likely the best one. That is assuming that it wasn't a cowboy that put the cable in because he had nothing else and 'couldn't be bothered'. Unfortunately, a lot of people DIY electrics, and I always recommend you seek and registered and competent electrician to check the continuity of the cable and that it's not going to turn your earths live if you just 'throw it in with the others'. I do ebb on the side of caution, ever since completing rewires and finding cable joints made with sellotape.