King in toga sent up criminal language (5)
I believe the answer is:
argot
'criminal language' is the definition.
'argot' can be an answer for 'language' (argot is a kind of language). I am not sure about the 'criminal' bit.
'king in toga sent up' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'in' means one lot of letters goes inside another.
'sent up' shows that the letters should be reversed in order.
'r' put within 'toga' is 'togra'.
'togra' written backwards gives 'ARGOT'.
Can you help me to learn more?
(Other definitions for argot that I've seen before include "Ancient Greek coin" , "Criminal jargon" , "Slang of a particular group, originally thieves" , "Slang for a groat (5)" , "Talk of thieves" .)