Bill enters revolutionary den of people (6)
I believe the answer is:
racial
'of people' is the definition.
(relating to a race or people)
'bill enters revolutionary den' is the wordplay.
'bill' becomes 'ac' (short for account).
'enters' is an insertion indicator.
'revolutionary' is a reversal indicator.
'den' becomes 'lair' (synonyms).
'lair' back-to-front is 'rial'.
'ac' inserted within 'rial' is 'RACIAL'.
(Other definitions for racial that I've seen before include "Pertaining to ethnic groups" , "Relating to ethnic divisions" , "Of particular people or colour, as with prejudice" , "Related to genetically distinguished groups of people" , "Relating to differences between nationalities" .)