Mean so much at first, but a little all round (7)
I believe the answer is:
awesome
'mean so much' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?
'first but a little all round' is the wordplay.
'first but' becomes 'we' (I can't justify this - if you can you should believe this answer much more).
'little' becomes 'some' (similar in meaning).
'all round' indicates putting letters inside.
'a'+'some'='asome'
'we' inserted within 'asome' is 'AWESOME'.
'at' is the link.
(Other definitions for awesome that I've seen before include "Simultaneously wonderful" , "Exceptionally good" , "Shocking" , "Frightening but impressive" , "Generating fear and wonder" .)