Train hooted, skirting theatre punctually (2,3,3)
I believe the answer is:
on the dot
'punctually' is the definition.
(eg 7 o'clock on the dot means 7 o'clock exactly)
'train hooted skirting theatre' is the wordplay.
'train' is an anagram indicator (to train can mean to grow something into a shape).
'skirting' indicates putting letters inside.
'theatre' becomes 'NT' (National Theatre).
'hooted' with letters rearranged gives 'ohedot'.
'ohedot' enclosing 'nt' is 'ON THE DOT'.
(Other definitions for on the dot that I've seen before include "Absolutely punctual" , "Sharp" , "At exactly the right time" , "Prompt" , "on precise place in line?" .)