Sees term out — a long one (8)
I believe the answer is:
semester
'a long one' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'sees term out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'sees'+'term'='seesterm'
'seesterm' is an anagram of 'SEMESTER'.
(Other definitions for semester that I've seen before include "(US) university term" , "Half-year academic course" , "Half-year university session" , "Steers me towards university term" , "study period" .)