For one short month, both master and monarch (3)
I believe the answer is:
mar
'for one short month' is the definition.
(short for March; 'short' indicates an abbreviation)
'both master and monarch' is the wordplay.
'both master' becomes 'ma' (Master of Arts. I am not sure about the 'both' bit.).
'and' is a charade indicator (letters next to each other).
'monarch' becomes 'R' (abbreviation for regina, e.g. in ER ).
'ma'+'r'='MAR'
Can you help me to learn more?
(Other definitions for mar that I've seen before include "Blight" , "Damage; month (abbr.)" , "Spoil something" , "when spring growth starts?" , "'Damage, impair (3)'" .)