British Rail director taking small measure is good idea (10)
I believe the answer is:
brainchild
'good idea' is the definition.
(thesaurus)
'british rail director taking small measure' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in BBC).
'director' becomes 'd' (abbreviation eg in film reviews).
'taking' is an insertion indicator.
'small measure' becomes 'inch' (small length measurement).
'rail'+'d'='raild'
'raild' placed around 'inch' is 'rainchild'.
'b'+'rainchild'='BRAINCHILD'
'is' acts as a link.
(Other definitions for brainchild that I've seen before include "Invention" , "wheeze" , "One's original idea" , "Original idea or work" , "Bright idea" .)