Telegram: TAXI LEFT HALFWAY THROUGH (5)
I believe the answer is:
cable
'telegram' is the definition.
(cable is a kind of telegram)
'taxi left halfway through' is the wordplay.
'taxi' becomes 'cab' (synonyms).
'left' becomes 'fled' (fleeing is a kind of leaving).
'halfway through' means to look at the middle letters.
The middle of 'fled' is 'le'.
'cab'+'le'='CABLE'
(Other definitions for cable that I've seen before include "Strong suspension bridge made of wire" , "Communication - wire - 200 yards in length" , "Naut. length; transmit" , "Liberal Democrat" , "Heavy chain or rope" .)