Country code found in post, mostly (6)
I believe the answer is:
malawi
'country' is the definition.
(I know that Malawi is a African country)
'code found in post mostly' is the wordplay.
'code' becomes 'law' (legal code).
'found in' indicates putting letters inside.
'post' becomes 'mail' (synonyms).
'mostly' means to remove the last letter.
'mail' with its final letter taken away is 'mai'.
'law' put within 'mai' is 'MALAWI'.
(Other definitions for malawi that I've seen before include "East African country" , "Republic" , "It was called Nyasaland" , "Former Nyasaland" , "Landlocked country of south central Africa" .)