In theory, time for a period of study (8,4)
I believe the answer is:
academic year
'period of study' is the definition.
'academic year' can be an answer for 'period' (academic year is a kind of period). I'm not sure about the 'of study' bit.
'in theory time' is the wordplay.
'in theory' becomes 'academic' (I am not sure about this - if you are sure you should believe this answer much more).
'time' becomes 'year' (year is a unit of time).
'academic'+'year'='ACADEMIC YEAR'
'for a' is the link.
Can you help me to learn more?
(Another definition for academic year that I've seen is " Period spanning terms at university".)