Family spokesman's reckoned he'd be staying with them (5,5)

'he'd be staying with them' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

'family spokesman's reckoned' is the wordplay.
'family' becomes 'house' (eg a royal house).
'spokesman's' shows a homophone (sound like) (how it might sound if spoken aloud).
'reckoned' becomes 'guessed' (guessing is a kind of reckoning).
'guessed' sounds like 'guest'.
'house'+'guest'='HOUSE GUEST'