Certain home study experiment up to it (13)
I believe the answer is:
incontestable
'certain' is the definition.
(thesaurus)
'home study experiment up to it' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'study' becomes 'con' (con can informally mean to study).
'experiment' becomes 'test' (test is a kind of experiment).
'up to it' becomes 'able' (up to it means able to do something).
'in'+'con'+'test'+'able'='INCONTESTABLE'
(Other definitions for incontestable that I've seen before include "Beyond dispute" , "sure" , "Without doubt" .)