Spar with senior tree (3,5)
I believe the answer is:
box elder
'tree' is the definition.
(box elder is a kind of tree)
'spar with senior' is the wordplay.
'spar' becomes 'box' (I've seen this in another clue).
'with' means one lot of letters go next to another.
'senior' becomes 'elder' (similar in meaning).
'box'+'elder'='BOX ELDER'
(Other definitions for box elder that I've seen before include "Maple" , "tree" .)