If down, model behaviour (9)
I believe the answer is:
supposing
'if' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for supposing that I've seen before include "Assuming to be true, suspecting" , "Hypothesising" , "Conjecturing, imagining" , "Fancying" .)