Triggering a single tax in effect (10)
I believe the answer is:
activation
'triggering a' is the definition.
I can't judge whether this definition defines the answer.
'single tax in effect' is the wordplay.
'single' becomes 'I' (one).
'tax' becomes 'vat' (UK Value Added Tax).
'in' means one lot of letters goes inside another.
'effect' becomes 'action' (action can mean a chemical effect).
'i'+'vat'='ivat'
'ivat' put within 'action' is 'ACTIVATION'.
Can you help me to learn more?
(Another definition for activation that I've seen is " Process of starting up (something)".)