A line spun by academic that's absolutely certain (6,2)
I believe the answer is:
nailed on
'absolutely certain' is the definition.
'nailed on' can be an answer for 'certain' (both can mean fixed or definite). I am not sure about the 'absolutely' bit.
'a line spun by academic' is the wordplay.
'spun' is an anagram indicator.
'by' is a charade indicator (letters next to each other).
'academic' becomes 'don' (type of academic - especially at Oxbridge).
'a'+'line'='aline'
'aline' is an anagram of 'naile'.
'naile'+'don'='NAILED-ON'
'that's' is the link.
Can you help me to learn more?
(Another definition for nailed on that I've seen is " horse so favoured?".)