Theoretical paper, second out for head of OU (8)
I believe the answer is:
notional
'theoretical' is the definition.
(similar in meaning)
'paper second out for head of ou' is the wordplay.
I cannot really see how this works, but
'paper' could be 'i' (UK newspaper) and 'i' is found within the answer.
'for' could be 'to' (eg both can mean 'in the opinion of') and 'to' is located in the remaining letters.
'of' could be 'o' and 'o' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for notional that I've seen before include "Imaginary - in on a lot (anag)" , "Unreal" , "Theoretic" , "Hypothetical, assumed to be correct" , "Theoretical" .)