Language of Inland Revenue is hard (5)
I believe the answer is:
irish
'language' is the definition.
(Irish is an example)
'inland revenue is hard' is the wordplay.
'inland revenue' becomes 'ir' (abbreviation for Inland Revenue).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'ir'+'is'+'h'='IRISH'
'of' acts as a link.
(Other definitions for irish that I've seen before include "Gaelic language" , "Relating to Ireland" , "From Dublin, perhaps" , "Passion" , "Hi, Sir, ar you a Celt?" .)