Short-tempered monarch caught in ridiculously basic lie (9)
I believe the answer is:
irascible
'short-tempered' is the definition.
(I know that short-tempered can be written as irascible)
'monarch caught in ridiculously basic lie' is the wordplay.
'monarch' becomes 'r' (abbreviation for rex, king in Latin).
'caught in' means one lot of letters goes inside another.
'ridiculously' indicates anagramming the letters (I've seen this in other clues).
'basic'+'lie'='basiclie'
'basiclie' anagrammed gives 'iascible'.
'r' inserted into 'iascible' is 'IRASCIBLE'.
(Other definitions for irascible that I've seen before include "Trigger-happy" , "Easily aroused to anger" , "Crabbed" , "Quick-tempered" , "Grouchy" .)