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