A noble brew of ale that's about right (4)
I believe the answer is:
'a noble' is the definition.
(earl is a kind of noble)
'brew of ale that's about right' is the wordplay.
'brew of' indicates anagramming the letters (I've seen this in other clues).
'that's about' indicates putting letters inside.
'right' becomes 'r' (common abbreviation).
'ale' with letters rearranged gives 'eal'.
'eal' enclosing 'r' is 'EARL'.
(Other definitions for earl that I've seen before include "Countesss husband" , "... Grey, fragrant Oriental tea" , "British peer and Dublin street" , "Peer (Br.)" , "A real peer" .)