It's prudent to be endlessly polite, I see (7)
I believe the answer is:
politic
'it's prudent' is the definition.
(I've seen this before)
'endlessly polite i see' is the wordplay.
'endlessly' means to remove the last letter.
'see' becomes 'C' (the word for the letter, according to Chambers).
'polite' with its final letter removed is 'polit'.
'polit'+'i'+'c'='POLITIC'
'to be' is the link.
(Other definitions for politic that I've seen before include "Diplomatic" , "Expedient" , "Prudent, discreet" , "Advisable" , "artful" .)