The field of ale brewing (3)
'the field' is the definition.
(I've seen this before)
'ale brewing' is the wordplay.
'brewing' indicates anagramming the letters.
'ale' with letters rearranged gives 'LEA'.
'of' is the link.
(Other definitions for lea that I've seen before include "authority for teachers", "Meadow (poetic)", "Open grassy land; river", "Field", "grazing area".)