For Buster Sims, I have a letter (7)
I believe the answer is:
missive
'a letter' is the definition.
('missive' can be a synonym of 'letter')
'for buster sims i' is the wordplay.
I cannot quite see how this works, but
an anagram of 'sims' is 'miss' which is located in the answer.
'i' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'have' acts as a link.
Can you help me to learn more?
(Other definitions for missive that I've seen before include "Letter" , "message" , "intelligence in this?" .)