Waterproof covering setter in rickety train (9)
I believe the answer is:
tarpaulin
'waterproof covering' is the definition.
(tarpaulin is a waterproof material)
'setter in rickety train' is the wordplay.
'setter' becomes 'paul' (Paul is a Guardian crossword setter).
'in' indicates putting letters inside.
'rickety' indicates an anagram.
'train' anagrammed gives 'tarin'.
'paul' inserted into 'tarin' is 'TARPAULIN'.
(Other definitions for tarpaulin that I've seen before include "waterproof material" , "Strong waterproof sheet" , "protective headgear" , "Waterproofed canvas" , "protective cover" .)