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Stupid
dad,
not
completely
stupid
dad
with
drug
injected
(9)
Stupid dad, not completely stupid dad with drug injected (9)
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The definition is at the beginning of the clue.
How the clue works
Stupid is the definition here. Take "father" (the formal word for dad) and drop its last letter ("not completely") to leave FATHE, then take "dad" with E (the drug ecstasy) injected into it and anagram the lot ("stupid") to give ADED, making FATHEADED.