Girl marrying outside key alpha group (7)
I believe the answer is:
brigade
'group' is the definition.
(I know that brigade is a more specific form of group)
'girl marrying outside key alpha' is the wordplay.
I cannot quite see how this works, but
'girl' could be 'de' and 'de' is found within the answer.
'key' could be 'g' (musical key) and 'g' is located in the answer.
'alpha' could be 'a' (phonetic alphabet: alpha, bravo, charlie etc.) and 'a' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for brigade that I've seen before include "Corps" , "Soldiers" , "Army grouping" , "Abridge (anag)" , "Rig a bed (anag)" .)