It's wonderful when mother joins Joe and Calvin (7)
I believe the answer is:
magical
'it's wonderful' is the definition.
(I've seen this before)
'mother joins joe and calvin' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'joins' says to put letters next to each other.
'joe' becomes 'GI' (GI Joe).
'and' says to put letters next to each other.
'calvin' becomes 'cal' (I can't explain this - if you can you should give a lot more credence to this answer).
'ma'+'gi'+'cal'='MAGICAL'
'when' is the link.
Can you help me to learn more?
(Other definitions for magical that I've seen before include "miraculous!" , "Occult, supernatural" , "Charming" , "Fascinating - occult" , "Enchanted" .)