Out loud, I refer to what the Observer must use (8)
I believe the answer is:
eyesight
'what the observer must use' is the definition.
I can't judge whether this defines the answer.
'out loud i refer' is the wordplay.
'out loud' indicates a 'sounds like' (homophone) clue.
'refer' becomes 'sight' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'i' is a homophone of 'eye'.
'eye'+'sight'='EYESIGHT'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for eyesight that I've seen before include "quality of viewing" , "Purpose of certain organs" , "Power of vision" , "Faculty of vision" , "Visual faculty" .)