Try Sunday Express? (6)
I believe the answer is:
strain
'try' is the definition.
(strain is a kind of try)
'sunday express?' is the wordplay.
'sunday' becomes 's'.
'express?' becomes 'train' (express is a kind of train).
's'+'train'='STRAIN'
(Other definitions for strain that I've seen before include "Muscle injury - tension" , "Strive; ancestry" , "Pass through a colander" , "Exert oneself to the utmost" , "Tension - stock" .)