Briefly, she offers an alternative (4)
I believe the answer is:
else
'an alternative' is the definition.
(I've seen this in another clue)
'briefly she offers' is the wordplay.
'briefly' means to remove the last letter.
'she' becomes 'Em'.
'offers' becomes 'lse' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'em' with its final letter taken off is 'e'.
'e'+'lse'='ELSE'
Can you help me to learn more?
(Other definitions for else that I've seen before include "Other than (what is under consideration)" , "Eels (anag)" , "Adverb meaning if not, otherwise" , "In addition; different" , "Alternatively" .)