It's out of habit, possibly, that one exclaims thus (3)
I believe the answer is:
bah
'one exclaims thus' is the definition.
I can't judge whether this definition defines the answer.
'it's out of habit possibly' is the wordplay.
'out of' indicates named letters should be taken away ('out of' can mean no longer having).
'possibly' is an anagram indicator.
'habit' with 'it' taken out is 'hab'.
'hab' is an anagram of 'BAH'.
'that' acts as a link.
Can you help me to learn more?
(Other definitions for bah that I've seen before include "''... Humbug'' Scrooge's cry" , "Comic expression of contempt" , "Exclamation of contempt or disgust - Scrooge used to say it" , "Exclamation of disgust or annoyance" , "''...! Humbug!'' (Scrooge)" .)