Clear about having left brief guest (6)
I believe the answer is:
caller
'guest' is the definition.
(I've seen this in another clue)
'clear about having left brief' is the wordplay.
'about' is an anagram indicator.
'having' is an insertion indicator.
'left brief' becomes 'l' (common abbreviation; 'brief' asks for an abbreviation).
'clear' with letters rearranged gives 'caler'.
'caler' placed around 'l' is 'CALLER'.
(Other definitions for caller that I've seen before include "One paying a brief visit" , "One visits" , "Person on the line" , "Visitor" , "Someone phoning" .)