A picture from the other side (showing the components) would help. Close-ups of the dropouts and lugs would also help give an impression of the quality of the frame.
It's obviously been well looked after, but old bikes, unless they're from one of the more famous marques or have collectable components, don't tend to fetch all that much money. I don't know if Clements are collectable. If the frame tubing is plain guage it'll be worth less than if it's butted. Check the 531 label at the top of the seat tube. Better frames had 531 stays and butted main tubes. Cheaper frames often used 531 just in the main tubes, with lesser steel for the stays.
There is something of a resurgence of interest in older steel frames to use as fixed gear bikes, so that might be one way to find a buyer, if it wou
ldn't break your Grandad's heart to see it go that way.
I got a 1975 Dawes Super Galaxy with fairly high end Suntour components, 531 double butted tubes, investment cast lugs for £230. That might be a good indicator of the price yours may go for------
here it is when originally got it without having done anything to it except pedals, rack and bag: