Girl seen by toff leaving Chernobyl (6)
I believe the answer is:
cheryl
'girl' is the definition.
'toff leaving chernobyl' is the wordplay.
'toff' becomes 'nob' (UK slang for a wealthy or upper-class person).
'leaving' indicates named letters should be taken away.
'chernobyl' with 'nob' taken away is 'CHERYL'.
'seen by' is the link.
(Other definitions for cheryl that I've seen before include "Woman" , "- Studer, US soprano" , "Girl" .)