Half four at lido, train regularly with greater confidence (1,8)
I believe the answer is:
a fortiori
'with greater confidence' is the definition.
(used for a claim one can be more confident about)
'half four at lido train regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'half'+'four'+'at'+'lido'+'train'='halffouratlidotrain'
The alternating letters of 'halffouratlidotrain' are 'A FORTIORI'.
(Other definitions for a fortiori that I've seen before include "with better grounds" , "For stronger reasons" , "stronger premises" , "more convincing reason" , "Latin phrase: With even stronger reason than that already accepted" .)