Heavy garment, one adopted by cottager at work (9)
I believe the answer is:
greatcoat
'heavy garment' is the definition.
'one adopted by cottager at work' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'adopted by' is an insertion indicator (I've seen this in other clues).
'at work' indicates an anagram.
'cottager' with letters rearranged gives 'greatcot'.
'a' inserted inside 'greatcot' is 'GREATCOAT'.
(Other definitions for greatcoat that I've seen before include "Long heavy outer garment" , "One may cover officer" , "costume?" , "Long overgarment" , "Petersham, perhaps" .)