A lovely person's back? Very good (4)
I believe the answer is:
mega
'very good' is the definition.
(informal term for excellent)
'a lovely person's back?' is the wordplay.
'lovely person' becomes 'gem' (I can't justify this - if you can you should give a lot more credence to this answer).
'back?' says the letters should be written in reverse (I've seen 'back' mean this).
'a'+'gem'='agem'
'agem' backwards is 'MEGA'.
Can you help me to learn more?
(Other definitions for mega that I've seen before include "Informally great or large" , "wicked" , "As brilliant" , "'Excellent, colloquially (4)'" , "Very large (prefix)" .)