Home Office head blocking just over 100 ordinary folk (3,6)
I believe the answer is:
hoi polloi
'ordinary folk' is the definition.
(Latin term for 'the many')
'home office head blocking just over 100' is the wordplay.
'home office' becomes 'ho' (UK government acronym).
'head' becomes 'poll' (poll is a now rare word for head or scalp).
'blocking' indicates putting letters inside.
'just over 100' becomes 'ioi' (resembles 101).
'poll' inserted within 'ioi' is 'ipolloi'.
'ho'+'ipolloi'='HOI POLLOI'
(Other definitions for hoi polloi that I've seen before include "Riff-raff" , "Proletariat" , "The common people - in Greece" , "The plain people in Greece" , "ordinary people" .)