Run second leg (4)
I believe the answer is:
spin
'run second leg' is the definition.
(I've seen this before)
'second leg' is the wordplay.
'second' becomes 's' (common abbreviation).
'leg' becomes 'pin' (pin is a kind of leg).
's'+'pin'='SPIN'
(Other definitions for spin that I've seen before include "Revolve, turn" , "Revolve, whirl" , "Slant put on news etc" , "Rotate quickly" , "Turn rapidly on axis" .)