Launch isn't off: one opening (9)
I believe the answer is:
instigate
'launch' is the definition.
Although both the answer and definition are verbs in their base form, I cannot understand how one could define the other.
'isn't off one opening' is the wordplay.
'off' indicates an anagram.
'one' becomes 'I' (Roman numeral).
'opening' becomes 'gate' (I have seen 'Movable opening ' mean 'gate' so perhaps 'opening' could also mean 'gate').
'isnt' is an anagram of 'inst'.
'inst'+'i'+'gate'='INSTIGATE'
Can you help me to learn more?
(Other definitions for instigate that I've seen before include "I taste gin to incite or urge" , "Bring about by incitement - its eating (anag)" , "Start" , "Prompt" , "Provoke" .)