Bird one encounters repeatedly (4)
I believe the answer is:
ibis
'bird' is the definition.
(ibis is a kind of bird)
'one encounters repeatedly' is the wordplay.
'one' becomes 'i' (Roman numeral).
'encounters repeatedly' becomes 'bis' (I am not sure about this - if you are sure you should believe this answer much more).
'i'+'bis'='IBIS'
Can you help me to learn more?
(Other definitions for ibis that I've seen before include "Wader once sacred in Egypt" , "Bird worshipped in Ancient Egypt" , "Wader with a long down-curved bill" , "Wading bird of warm regions" , "Bird sacred in Ancient Egypt" .)