Field Australian league's sides, at first (3)
I believe the answer is:
lea
'field' is the definition.
(lea can mean a meadow or field of grass)
'australian league's sides at first' is the wordplay.
'australian' becomes 'A' (common abbreviation).
'sides' suggests removing the centre (only the letters on the sides are used).
'at first' means one lot of letters go next to another.
'league' with its middle taken out is 'le'.
'a' put after 'le' is 'LEA'.
(Other definitions for lea that I've seen before include "authority for teachers" , "grazing area" , "area of grassland" , "River of east London" , "Open pasture" .)