Confirm most obese female is to leave (6)
I believe the answer is:
attest
'confirm' is the definition.
(thesaurus)
'most obese female is to leave' is the wordplay.
'most obese' becomes 'fattest' ('fat' can be similar in meaning to 'obese').
'female' becomes 'f' (abbreviation).
'is to leave' suggests deleting specific letters.
'fattest' with 'f' taken away is 'ATTEST'.
(Other definitions for attest that I've seen before include "Demonstrate" , "Give evidence in a court of law" , "Act as witness" , "Certify or bear witness" , "Vouch for" .)