In the moon, perhaps, but she’s heavenly (5)
I believe the answer is:
diana
'in the moon perhaps' is the definition.
I can't tell whether this defines the answer.
'she's heavenly' is the wordplay.
'she' becomes 'di'.
'heavenly' becomes 'ana' (I can't explain this - if you can you should give a lot more credence to this answer).
'di'+'ana'='DIANA'
'but' acts as a link.
Can you help me to learn more?
(Other definitions for diana that I've seen before include "female" , "She separated from Charles" , "Roman goodess of hunting or tragic English princess" , "Woman" , "British princess" .)