Observe weary exhalation prior to head teacher presenting vision (8)
I believe the answer is:
eyesight
'vision' is the definition.
(eyesight is your vision)
'observe weary exhalation prior to head teacher' is the wordplay.
'observe' becomes 'eye' (eye can mean to observe or see).
'weary exhalation' becomes 'sigh' (I've seen this before).
'prior to' says to put letters next to each other.
'head' says to take the initial letters.
The first letter of 'teacher' is 't'.
'eye'+'sigh'+'t'='EYESIGHT'
'presenting' is the link.
(Other definitions for eyesight that I've seen before include "Yes, eight required for vision" , "Power of seeing" , "Visual faculty" , "Requirement to look" , "View" .)