Giant moon, one missed by artist (5)
I believe the answer is:
titan
'giant moon' is the definition.
(moon of Saturn named after mythological giant)
'one missed by artist' is the wordplay.
I cannot quite see how this works, but
'one' could be 'an' ('a' and 'an' can mean 'one') and 'an' is located in the answer.
The remaining letters 'tit' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for titan that I've seen before include "one of great importance" , "Old Greek god or very strong person" , "Prometheus, for instance" , "Taint in a great man" , "One of elder gods defeated by Zeus" .)