He could be simply saintly (5)
I believe the answer is:
simon
'he could be simply saintly' is the definition.
The definition suggests a singular noun which matches the answer.
'he could be simply' is the wordplay.
'he' becomes 'Sim'.
'could be simply' becomes 'on' (I can't explain this - if you can you should give a lot more credence to this answer).
'sim'+'on'='SIMON'
Can you help me to learn more?
(Other definitions for simon that I've seen before include "Peter (NT)" , "Sir -- Rattle, conductor" , "he says it's a game" , "Simple pieman" , "A simple character in nursery rhyme" .)