Count on keys on post's top extremity (6)
I believe the answer is:
depend
'count on' is the definition.
(thesaurus)
'keys on post's top extremity' is the wordplay.
'keys' becomes 'de' (I've seen this in other clues).
'on' says to put letters next to each other.
'top' suggests taking the first letters.
'extremity' becomes 'end' (end is a kind of extremity**).
The initial letter of 'posts' is 'p'.
'de'+'p'+'end'='DEPEND'
(Other definitions for depend that I've seen before include "Rest" , "Rely or hang on" , "Be contingent upon" , "Dangle - rely" , "Put trust (in)" .)