Beware of taking Telegraph on vacation - turn page to reveal transport advice (4,3,3)
I believe the answer is:
mind the gap
'transport advice' is the definition.
'beware of taking telegraph on vacation turn page' is the wordplay.
'beware of' becomes 'mind' ('mind' can be a synonym of 'beware of').
'taking' says to put letters next to each other.
'on vacation' means to remove the middle letters.
'turn' says the letters should be written backwards.
'telegraph' with its middle removed is 'th'.
'page' in reverse letter order is 'egap'.
'mind'+'th'+'egap'='MIND THE GAP'
'to reveal' is the link.
(Another definition for mind the gap that I've seen is " Warning to those leaving train".)