Drone monotonously the length of Mass (5)
I believe the answer is:
thrum
'drone monotonously' is the definition.
'thrum' can be an answer for 'drone' (I've seen this before). I'm not sure about the 'monotonously' bit.
'length of mass' is the wordplay.
'length of' becomes 'thru' (I can't justify this - if you can you should believe this answer much more).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'thru'+'m'='THRUM'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for thrum that I've seen before include "Strum; thread end" , "Monotonously repeat" , "Make a rhythmic humming sound" , "Drum with fingers - bits of coarse yarn" , "Single thread" .)