Visitors speculate about temperature (6)
I believe the answer is:
guests
'visitors' is the definition.
(I know this)
'speculate about temperature' is the wordplay.
'speculate' becomes 'guess' (guessing is a kind of speculating).
'about' is an insertion indicator.
'temperature' becomes 't' (abbreviation).
'guess' enclosing 't' is 'GUESTS'.
(Other definitions for guests that I've seen before include "Hosts entertain them" , "Those invited" , "Visitors to whom hospitality is extended" , "House or hotel visitors" , "Invitees or hotel customers" .)