Monotonous drone preceding tattoo (7)
I believe the answer is:
humdrum
'monotonous' is the definition.
(I know that monotonous can be written as humdrum)
'drone preceding tattoo' is the wordplay.
'drone' becomes 'hum' (drone can mean to make a humming sound).
'preceding' is a charade indicator (letters next to each other).
'tattoo' becomes 'drum'.
'hum'+'drum'='HUMDRUM'
(Other definitions for humdrum that I've seen before include "Everyday, boring" , "without incident" , "Ordinary - monotonous" , "Far from exciting" , "Mundane, commonplace" .)