Senior pupil is ideal if frame of reference is reversed (7)
I believe the answer is:
prefect
'senior pupil' is the definition.
(I've seen this before)
'ideal if frame of reference is reversed' is the wordplay.
I cannot really understand how this works, but
'reference' could be 'ref' (I've seen this in other clues) and 'ref' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'is' is the link.
Can you help me to learn more?
(Other definitions for prefect that I've seen before include "School monitor" , "School senior" , "Chief officer (in France) - senior pupil" , "Senior pupil with powers" , "person in authority" .)