His Excellency probes Byzantine prince in disguise (8)
I believe the answer is:
encipher
'disguise' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how one could define the other.
'his excellency probes byzantine prince' is the wordplay.
'his excellency' becomes 'HE' (abbreviation).
'probes' indicates putting letters inside (probe can mean to poke or prod into).
'byzantine' indicates an anagram (byzantine means complex or confusing).
'prince' anagrammed gives 'encipr'.
'he' inserted within 'encipr' is 'ENCIPHER'.
'in' is the link.
Can you help me to learn more?
(Other definitions for encipher that I've seen before include "Convert to coded form" , "Convert ordinary language into code" , "Put into code" .)