Pater's home, regrettably in mood! (10)
I believe the answer is:
atmosphere
'mood' is the definition.
(atmosphere is a kind of mood)
'pater's home regrettably' is the wordplay.
'regrettably' is an anagram indicator.
'paters'+'home'='patershome'
'patershome' is an anagram of 'ATMOSPHERE'.
'in' is the link.
(Other definitions for atmosphere that I've seen before include "We are always enveloped in this" , "Spirit" , "Pervading tone or mood" , "climate" , "Feeling" .)