Pay back half of rent? Absolutely (7)
I believe the answer is:
requite
'pay back' is the definition.
(thesaurus)
'half of rent? absolutely' is the wordplay.
'half of' indicates taking half.
'absolutely' becomes 'quite' ().
'rent' halved is 're'.
're'+'quite'='REQUITE'
(Other definitions for requite that I've seen before include "Appropriately, give back" , "Quieter (anag.)" , "Make return" .)