Run summer league at first (6)
I believe the answer is:
ladder
'run' is the definition.
('ladder' can be a synonym of 'run')
'summer league at first' is the wordplay.
'summer' becomes 'adder' (someone who adds or does sums).
'league' becomes 'l' (abbreviation).
'at first' is a charade indicator (letters next to each other).
'adder' after 'l' is 'LADDER'.
(Other definitions for ladder that I've seen before include "Support for climber" , "sometimes with companion" , "Climbing aid" , "Climbing frame; damage tights" , "Companionway" .)