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 am not sure about the 'the moon' bit.
'should appear above the horizon' is the wordplay.
'should' becomes 'theo' (I can't justify 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" .)