Leave out four and five at the end of letter (7)
I believe the answer is:
missive
'letter' is the definition.
(I know that letter can be written as missive)
'leave out four and five at the end' is the wordplay.
'leave out' becomes 'miss' (synonyms).
'four' becomes 'IV' (Roman numerals for four).
'and' says to put letters next to each other.
'at the end' indicates one should take the final letters.
The final letter of 'five' is 'e'.
'miss'+'iv'+'e'='MISSIVE'
'of' is the link.
(Other definitions for missive that I've seen before include "Letter" , "intelligence in this?" , "message" .)