Belonging to those at the Inland Revenue (5)
I believe the answer is:
their
'belonging to those' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'at the inland revenue' is the wordplay.
'at' says to put letters next to each other.
'inland revenue' becomes 'ir' (abbreviation for Inland Revenue).
'the' next to 'ir' is 'THEIR'.
Can you help me to learn more?
(Other definitions for their that I've seen before include "Belonging to those people" , "belonging to an individual" , "Those people's" , "Belonging to him (perhaps wrongly)" , "Relating to them" .)