Think quietly about drone (6)
I believe the answer is:
ponder
'think' is the definition.
(pondering is a kind of thinking)
'quietly about drone' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet**).
'about' is an anagram indicator.
'drone' is an anagram of 'onder'.
'p'+'onder'='PONDER'
(Other definitions for ponder that I've seen before include "Weigh in the mind" , "Muse" , "Think deeply about, ruminate" , "Consider deeply" , "Meditate - deliberate" .)