Mother curtailed baby's insubordination (6)
I believe the answer is:
mutiny
'insubordination' is the definition.
(thesaurus)
'mother curtailed baby's' is the wordplay.
'mother' becomes 'mum' (informal term for mother).
'curtailed' means to remove the last letter.
'baby' becomes 'tiny' (I've seen this in another clue).
'mum' with its last letter taken off is 'mu'.
'mu'+'tiny'='MUTINY'
(Other definitions for mutiny that I've seen before include "Rebellion as on Bligh's Bounty" , "Open rebellion against constituted authority" , "Open rebellion by sailors or soldiers" , "Insurrection" , "Military rebellion" .)