Act in a fitting manner (3,2,2)
I believe the answer is:
try it on
'manner' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'act in a fitting' is the wordplay.
'act' becomes 'it' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'a fitting' becomes 'tryon' ('try-on' can be a synonym of 'fitting').
'it' put into 'tryon' is 'TRY IT ON'.
Can you help me to learn more?
(Other definitions for try it on that I've seen before include "See how far one may go" , "trick" , "Be audacious" , "Attempt a fitting" , "Attempt to be provocative (colloq.)" .)