Fit in extra guest (4)
I believe the answer is:
ague
'fit' is the definition.
(I've seen this before)
'in extra guest' is the wordplay.
'in' indicates a hidden word.
'AGUE' is hidden in the letters of 'extra guest'.
(Other definitions for ague that I've seen before include "Hot and cold fever" , "Malaria, for example" , "Fever (archaic)" , "Malarial fever; chill" , "Illness with fever and shivering, malaria say" .)