One of the CID to come down and alight on platform (7)
I believe the answer is:
detrain
'alight on platform' is the definition.
(to detrain is to step off a train)
'one of the cid to come down' is the wordplay.
'one of the cid' becomes 'det' (short for detective, from the CID).
'to come down' becomes 'rain' (raining is a kind of coming down).
'det'+'rain'='DETRAIN'
'and' is the link.
(Other definitions for detrain that I've seen before include "leave coach behind" , "leave car" , "get out of carriage" , "Get off at a station" , "leave Orient Express, perhaps" .)