Let down badly in paid post (10)
I believe the answer is:
disappoint
'let down' is the definition.
('disappoint' can be a synonym of 'let down')
'badly in paid post' is the wordplay.
'badly' indicates an anagram.
'in'+'paid'+'post'='inpaidpost'
'inpaidpost' is an anagram of 'DISAPPOINT'.
(Other definitions for disappoint that I've seen before include "Fail to satisfy" , "Let down" .)