Brilliant girl without name (4)
I believe the answer is:
mega
'brilliant' is the definition.
(old-fashioned UK slang for 'very good')
'girl without name' is the wordplay.
'girl' becomes 'g'.
'without' means one lot of letters goes inside another (some letters go outside or 'without' others).
'name' becomes 'mea' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'g' inserted into 'mea' is 'MEGA'.
Can you help me to learn more?
(Other definitions for mega that I've seen before include "ace" , "Great - denoting a million" , "wicked" , "extremely good" , "'Excellent, colloquially (4)'" .)