Deceitful person cheers king retreating (3)
I believe the answer is:
rat
'deceitful person' is the definition.
'rat' can be an answer for 'person' (rat is a kind of person). I'm not sure about the 'deceitful' bit.
'cheers king retreating' is the wordplay.
'cheers' becomes 'ta' (both can mean thank you).
'king' becomes 'r' (abbreviation of Latin rex).
'retreating' shows that the letters should be reversed in order.
'ta'+'r'='tar'
'tar' reversed gives 'RAT'.
Can you help me to learn more?
(Other definitions for rat that I've seen before include "Domestic rodent" , "one who betrayed" , "Treacherous person" , "one not going down with ship" , "Tell-tale" .)