Jersey, say, worn by everyone is green (6)
I believe the answer is:
callow
'is green' is the definition.
(thesaurus)
'jersey say worn by everyone' is the wordplay.
'jersey say' becomes 'cow' (Jersey cow is an example).
'worn by' means one lot of letters goes inside another (I've seen this in other clues).
'everyone' becomes 'all' (I've seen this before).
'cow' placed around 'all' is 'CALLOW'.
(Other definitions for callow that I've seen before include "Inexperienced or unsophisticated, as a youth say" , "Immature and inexperienced, like youth" , "23 [BRIGHTONIAN] actor" , "Simon ---, actor; immature" , "Green" .)