Santa's terrible habit, poor thing, when cooped up? (14)
I believe the answer is:
claustrophobia
'thing when cooped up?' is the definition.
(discomfort experienced in a small space)
'santa's terrible habit poor' is the wordplay.
'santa' becomes 'claus' (Santa Claus).
'terrible' is an anagram indicator.
'habit'+'poor'='habitpoor'
'habitpoor' with letters rearranged gives 'trophobia'.
'claus'+'trophobia'='CLAUSTROPHOBIA'
(Other definitions for claustrophobia that I've seen before include "Fear of confinement" , "major worry over space" , "feeling of being badly stuck in a corner?" , "Fear of confined spaces" .)