One should be engaged in diplomacy, it's implied (5)
I believe the answer is:
tacit
'implied' is the definition.
(synonyms)
'one should be engaged in diplomacy' is the wordplay.
'one should be' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'engaged in' indicates putting letters inside.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'i' put inside 'tact' is 'TACIT'.
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for tacit that I've seen before include "without speaking" , "Implicitly understood" , "Unvoiced" , "Implied -- inferred" , "Wordless" .)