Doubt at regular intervals before sitting — that’s the thing (9)
I believe the answer is:
obsession
'the thing' is the definition.
(I've seen this before)
'doubt at regular intervals before sitting' is the wordplay.
'at regular intervals' means one should take alternating letters.
'before' says to put letters next to each other.
'sitting' becomes 'session' (eg a parliamentary session).
The alternate letters of 'doubt' are 'ob'.
'ob'+'session'='OBSESSION'
'that's' acts as a link.
(Other definitions for obsession that I've seen before include "Persistent preoccupation" , "Enthusiasm" , "Continual preoccupation" , "All-absorbing idea" , "Extreme interest" .)