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' indicates putting letters inside (I've seen this in other clues).
'at work' is an anagram indicator.
'cottager' is an anagram of 'greatcot'.
'a' inserted within '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" .)