All the London railway stations :
Clapton is on the London Overground, and is not accessible to wheelchair travel due to there being stairs between the street and the platforms. See pictures and further accessible transport in London detail from links below.
National Rail : London Overground
AccessAble Access Guide :
Accessibility friendly : There are steps to all platforms, making this station inaccessible.
General description : See wiki
More comment : National Rail map & guide