Check for cheats on public transport (9)
I believe the answer is:
constrain
'check' is the definition.
(constraining is a kind of checking)
'cheats on public transport' is the wordplay.
'cheats' becomes 'cons' (con is a kind of cheat).
'on' says to put letters next to each other.
'public transport' becomes 'train' (train is a kind of public transport).
'cons'+'train'='CONSTRAIN'
'for' acts as a link.
(Other definitions for constrain that I've seen before include "Confine, limit" , "Compel; constrict" , "Lock up" , "Compel - limit" , "Check" .)