Suppose the moon should appear above the horizon (8)
I believe the answer is:
theorise
'suppose the moon' is the definition.
'theorise' can be an answer for 'suppose' (synonyms). I'm unsure of the 'the moon' bit.
'should appear above the horizon' is the wordplay.
'should' becomes 'theo' (I can't explain this - if you can you should give a lot more credence to this answer).
'appear above the horizon' becomes 'rise' (I've seen this in another clue).
'theo'+'rise'='THEORISE'
Can you help me to learn more?
(Other definitions for theorise that I've seen before include "Form of hypothesis or conjecture" , "Express ideas" , "Conjecture, hypothesise" , "Suppose" , "Speculate by reasoning" .)