One prepared for posting to West Country, ultimately amenable (4)
I believe the answer is:
easy
'amenable' is the definition.
(thesaurus)
'one prepared for posting to west country ultimately' is the wordplay.
'one prepared for posting' becomes 'SAE' (short for stamp-addressed envelope).
'to west' is a reversal indicator (the letters are written right to left).
'ultimately' says to take the final letters.
The last letter of 'country' is 'y'.
'sae' backwards is 'eas'.
'eas'+'y'='EASY'
(Other definitions for easy that I've seen before include "Presenting few difficulties" , "Obliging; go carefully" , "Gentle - light" , "Straightforward" , "Simple - not fussy" .)