Yob one encountered in fair (3)
I believe the answer is:
oik
'yob' is the definition.
(I've seen this before)
'one encountered in fair' is the wordplay.
'one' becomes 'i' (Roman numeral).
'encountered in' indicates putting letters inside.
'fair' becomes 'ok'.
'i' put into 'ok' is 'OIK'.
(Other definitions for oik that I've seen before include "Torah scroll cupboard" , "anti-social type" , "Yob" , "Twerp" , "Brat" .)