Watertight evidence proceeded to trap one at the top (9)
I believe the answer is:
rainproof
'watertight' is the definition.
(thesaurus)
'evidence proceeded to trap one at the top' is the wordplay.
'evidence' becomes 'proof' (evidence proves something).
'proceeded' becomes 'ran' (I am not sure about this - if you are sure you should believe this answer much more).
'to trap' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'at the top' says to put letters next to each other (in a down clue, some letters go above others).
'ran' placed around 'i' is 'rain'.
'proof' put after 'rain' is 'RAINPROOF'.
Can you help me to learn more?
(Other definitions for rainproof that I've seen before include "coat" , "Impervious" , "protected from storms?" , "Like protective cover" , "Macintosh is" .)