Appropriate for Rob to return argument (6)
I believe the answer is:
borrow
'appropriate' is the definition.
(thesaurus)
'rob to return argument' is the wordplay.
'to return' says the letters should be written backwards.
'argument' becomes 'row' (I've seen this before).
'rob' reversed gives 'bor'.
'bor'+'row'='BORROW'
'for' acts as a link.
(Other definitions for borrow that I've seen before include "Get the temporary use of, but give back" , "In library, ask for" , "Take on loan" , "Temporarily adopt" , "appropriate for a time" .)