Sugar Cane, one of Clapham's more affordable bars, has Happy Hour for a few hours every weekday, and I would recommend any of the drinks involving tropical juice, which go down a treat. And £5 for a fairly well-brewed cocktail isn't something I can argue with really, particularly as Happy Hour in most London bars equated to Buy One Get One Free, or two cocktails for a tenner. This is much better because there are no constraints on what your other drink has to be!
Sugar Cane also has a good ambience (although the lights were a bit too dim), well-cleaned toilets, and boast a decent food menu. I've been to Sugar Cane about five times and the service has been hit-and-miss, the worst being a woman who took an absolute lifetime to make a simple cocktail, and didn't crack a smile once. However, if you get the man who appears to run the place, then you're service will be good - he knows what he's doing.
If you're around Clapham, worth a visit.