Leg I planted without assistance (2,4,3)
I believe the answer is:
on ones own
'without assistance' is the definition.
(on one's own means without help or assistance)
'leg i planted' is the wordplay.
'leg' becomes 'on' (cricket term, fielding positions).
'i' becomes 'one' (Roman numeral for one).
'planted' becomes 'sown' (similar in meaning).
'on'+'one'+'sown'='ON ONES OWN'
(Other definitions for on ones own that I've seen before include "Independently" , "Helpless" , "Alone" .)