Relate to one in apparent upheaval (9)
I believe the answer is:
appertain
'relate' is the definition.
(I've seen this before)
'one in apparent upheaval' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' means one lot of letters goes inside another.
'upheaval' indicates anagramming the letters.
'apparent' anagrammed gives 'appertan'.
'i' placed inside 'appertan' is 'APPERTAIN'.
'to' is the link.
(Other definitions for appertain that I've seen before include "Belong, relate (to)" , "Bear on" , "Be relevant to" , "Concern" .)