Two mums shunning monarch: it's huge (7)
I believe the answer is:
mammoth
'huge' is the definition.
(synonyms)
'two mums shunning monarch' is the wordplay.
'two mums' can mean two substitutions for 'mum'.
'mum' becomes 'mam' (I can't justify this - if you can you should believe this answer much more).
'mum' becomes 'mother' (I've seen this before).
'shunning' is a deletion indicator.
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'mother' with 'er' taken out is 'moth'.
'mam'+'moth'='MAMMOTH'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for mammoth that I've seen before include "Large, woolly ice-age beast" , "Large extinct elephant" , "Immense" , "Prehistoric beast like hairy elephant" , "Huge" .)