Say when northbound train's beginning to enter platform (5)
I believe the answer is:
stage
'platform' is the definition.
(I know that stage is a type of platform)
'say when northbound train's beginning to enter' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'northbound' says the letters should be written backwards.
'beginning to' indicates taking the first letters.
'enter' means one lot of letters goes inside another.
The initial letter of 'trains' is 't'.
'eg'+'as'='egas'
'egas' reversed gives 'sage'.
'sage' enclosing 't' is 'STAGE'.
(Other definitions for stage that I've seen before include "division" , "The boards" , "Produce" , "Elevated platform" , "Platform for actors" .)