Unprincipled about runs in first season (6)
I believe the answer is:
vernal
'in first season' is the definition.
(vernal means relating to spring)
'unprincipled about runs' is the wordplay.
'unprincipled' becomes 'venal' (venal means corrupt or bribable).
'about' is an insertion indicator.
'runs' becomes 'r' (cricket abbreviation).
'venal' placed around 'r' is 'VERNAL'.
(Other definitions for vernal that I've seen before include "Occuring in spring" , "Of or occurring in spring" , "Fresh" , "Seasonal breaks" , "Relating to Spring" .)