The extent of the telegram showing nautical measure (6,6)
I believe the answer is:
cables length
'measure' is the definition.
(cable's length is a kind of measure)
'the extent of the telegram showing nautical' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
'telegram' could be 'cable' (cable is a kind of telegram) and 'cable' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cables length that I've seen before include "from plug to device?" , "Some six-hundred feet" .)