Hypothetically (2,6)
'hypothetically' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for in theory that I've seen before include "In principal" , "according to the assumed facts" , "Ideally" , "On paper" .)
(Other definitions for what if that I've seen before include "hypothetical question" , "Hypothetical situation" , "How would things go when" , "How would it be when (something happened?)" , "Suppose" .)