Conjecture about tense invitees (6)
I believe the answer is:
guests
'invitees' is the definition.
('guest' can be a synonym of 'invitee')
'conjecture about tense' is the wordplay.
'conjecture' becomes 'guess' (synonyms).
'about' indicates putting letters inside.
'tense' becomes 't' (abbreviation used in many dictionaries).
'guess' enclosing 't' is 'GUESTS'.
(Other definitions for guests that I've seen before include "Invitees or hotel customers" , "Those invited" , "House or hotel visitors" , "Folk receiving hospitality" , "Visitors to whom hospitality is extended" .)