Harmless drunk being nice -- at first (6)
I believe the answer is:
benign
'harmless' is the definition.
(I've seen this before)
'drunk being nice at first' is the wordplay.
'drunk' is an anagram indicator.
'at first' suggests taking the first letters.
The initial letter of 'nice' is 'n'.
'being' anagrammed gives 'benig'.
'benig'+'n'='BENIGN'
(Other definitions for benign that I've seen before include "Kindly; not harmful" , "Innocuous, not malignant" , "Favourable" , "Medically curable" , "Salubrious" .)