Mum’s the word in strike planning! (13)
I believe the answer is:
masterminding
'planning' is the definition.
(masterminding is a kind of planning)
'mum's the word in strike' is the wordplay.
'mum's' becomes 'ma's' (both can mean mother).
'the word' becomes 'term' (term is a kind of word).
'strike' becomes 'ding' (I can't explain this - if you can you should believe this answer much more).
'mas'+'term'+'in'+'ding'='MASTERMINDING'
Can you help me to learn more?
(Another definition for masterminding that I've seen is " Organising".)