Work outside arbiter's plant (7)
I believe the answer is:
trefoil
'plant' is the definition.
(I know that trefoil is a type of plant)
'work outside arbiter's' is the wordplay.
'work' becomes 'toil' (toiling is a kind of working).
'outside' means one lot of letters goes inside another (some letters go outside others).
'arbiter' becomes 'ref' (I've seen this in other clues).
'toil' placed around 'ref' is 'TREFOIL'.
(Other definitions for trefoil that I've seen before include "Small plant with three-lobed leaves" , "A flower; three-lobe ornament" , "Three-lobed leaf" , "Clover leaf" , "Plant with clover-like leaves" .)